So I'm typing up a VGC primer for two friends that are likely to be tagging along with me to Indianapolis this year. Can any of you guys give this a read and let me know if I missed anything important or if anything is off-base?
I'm going to be adding a list and analysis of commonly-used Pokemon in the regionals as well, including one for every permitted Uber. This will be just as much for my own teambuilding reference as it is for theirs.
DOUBLE BATTLE TIPS
Teach Protect/Detect to anything that can sacrifice a moveslot for it, and particularly anything with mediocre defenses (except Choice item users). Since you have 8 moves at your disposal at any given time in doubles instead of 4, this is usually feasible to do without losing too much attack type coverage. Protecting is typically the most common defensive action in doubles. Switching is not as common and entry hazard attacks (Stealth Rock, Spikes, Rapid Spin) are not as useful.
Use leads that can either attack out of the gate or take very little time to set-up (one turn maximum IMO). Do not use complicated set-ups that can be easily deduced and foiled by your opponent. Try to prevent as many bad matchups for your leads as possible, as you do not have the luxury of switching frequently without often paying a heavy price for it.
When playing doubles, be able to identify which of your targets are likely to be attacking, setting up, Protecting, or serving as decoys on any given turn, and act accordingly. This applies doubly to leads, which are often the ones used to set up any global effects. If you see a relatively weak Pokemon being used by your opponent that is not likely to be doing any direct damage, try to figure out what it's there for and how threatening its abilities are to your team. Prioritize your actions according to how threatening such Pokemon are and how easily you can KO or disable them.
Be aware of doubles-specific attributes for various moves. Earthquake, Surf, Discharge, Explosion, and other moves hit all Pokemon on the field, including the user's partner. When using such moves, make sure that the rest of your team can take that hit, either with heavy resistances, Protect, or special abilities (Levitate, Water Absorb, Volt Absorb, etc). Blizzard, Water Spout, Rock Slide, Heat Wave, and others hit both opponents but not the partner.
Furthermore, an attack that hits multiple opponents only hits with 75% of its full strength. The exception is when there is only one target left standing. This applies even when one target is KO'd in the middle of a turn. Say you use Water Spout against a Groudon and Zapdos combo, and Zapdos is the faster of the two. Water Spout will hit Zapdos with 75% strength, but if Zapdos is KO'd, then Water Spout will hit Groudon with the full 100% of its power.
Be aware of moves that are only used in doubles. Follow Me causes all targetted attacks to hit the user, and the user of this move often serves as a decoy so that its partner can set up freely. It can be foiled by using moves that target multiple opponents. Follow Me can only be learned by Smeargle, Clefable, Togekiss, Lucario, Electivire, Furret, Mr. Mime, Pikachu, and their evolution lines.
Helping Hand increases the strength of the partner's attack by 50% and has higher speed priority than Protect. It is learned by several more Pokemon than Follow Me, but some notable ones include: Arcanine, Azelf, Blissey, Breloom, Cherrim, Clefable, Cresselia, Electivire, Gallade, Hitmontop, Infernape, Jumpluff, Latias, Latios, Machamp, Magmortar.
Moves requring a charge turn (Fly, SolarBeam, Dig, Dive, Shadow Force, etc.) are heavily discouraged in single battles because they telegraph your move to the opponent and give him a free opportunity to switch to something with an advantageous matchup. This doesn't quite apply to doubles, as switching is less frequent in general and it's impossible to tell which Pokemon the user is targetting until the move actually hits. Giratina's signature move, Shadow Force, is particularly notable because it cannot be blocked by Protect or Detect.
There are other moves seldomly used in singles that have utility in doubles. Dream Eater can be viable if the target is put to sleep earlier in the same turn. Swagger can be used on an Own Tempo partner to double its attack stat without being confused. Skill Swap and Worry Seed can be used on a partner to dispose of a disadvantageous ability (such as Regigigas's Slow Start) or to gain an advantageous one (such as a Sheer Cold Articuno with No Guard). Notable Skill Swap users include: Mewtwo, Lugia, Azelf, Blissey, Bronzong, and Cresselia. Still other useful support moves include Taunt, Encore, Psych Up (usable by several powerful legendaries), Gastro Acid, and Tailwind.
WEATHER EFFECTS
Weather is more popular in doubles than singles in general, and since Kyogre and Groudon are permitted for use in VGC 2010, whoever controls the weather is often in control of the match. Make sure that your team is capable of functioning well in multiple types of weather.
Rain: Water attacks do 1.5x damage. Fire attacks and SolarBeam do 0.5x damage. Thunder has 100% accuracy instead of 70%. Weather-dependent healing moves heal 25% HP instead of 50%. Pokemon w/ Rain Dish and Dry Skin heal themselves a little at the end of every turn.
Hail: All non-Ice Pokemon lose 1/16th of their HP each turn. Blizzard has 100% accuracy instead of 70%. Weather-dependent healing moves heal 25% HP instead of 50%. Pokemon w/ Ice Body heal themselves a little at the end of every turn.
Sun: Fire attacks do 1.5x damage. Water attacks do 0.5x damage. SolarBeam requires no charge turn. Thunder is 50% accurate. Weather-dependent healing moves heal 67% HP. Pokemon cannot be frozen. Pokemon with Chlorophyll have double speed. Pokemon with Flower Gift and their partners receive stat boosts. Pokemon with Dry Skin / Solar Power lose 1/8th HP every turn.
Sandstorm: All non-Rock/Ground/Steel Pokemon lose 1/16th of their HP each turn. Rock Pokemon earn a 50% special defense boost. SolarBeam does 0.5x damage. Weather-dependent healing moves heal 25% HP.
SPEED, TRICK ROOM, AND PRIORITY MOVES
Offense-driven metagames (of which VGC 2010 most certainly is) are heavily decided by speed. Whoever moves first on any given turn usually has a significant advantage. Try to glean as much information from the field as possible to determine the speed order of all Pokemon involved. When any multi-target effect/attack is used, the status messages will appear in order from fastest target to slowest target. If Earthquake hits all 3 other Pokemon, the fastest Pokemon will take damage first. If Intimidate activates, it will affect the fastest target first. When a hail storm is in play, it will affect the fastest target first.
Trick Room reverses the speed of all Pokemon for 5 turns (including the turn it is used, meaning that it is only really utilized for 4 turns). Pokemon can take advantage of Trick Room by running -Speed natures, allocating no Speed EVs, and using hold items such as Iron Ball to cut speed in half. This allows them to allocate EVs in other stats without worrying about speed.
Priority levels are not affected by Trick Room. A fast Pokemon using Ice Shard (+1 priority) will still attack before a slow Pokemon using Blizzard (0 priority). A slow Pokemon using Ice Shard would move before a fast Pokemon using Ice Shard, however. For this reason, it's a good idea to have several priority moves distributed throughout your team. Fake Out, Mach Punch, Sucker Punch, Ice Shard, Shadow Sneak, and ExtremeSpeed are good examples.
If two auto-weather Pokemon (Kyogre, Abomasnow, Groudon, Tyranitar, Hippowdon) are sent into play on the same turn, the Pokemon that moves last is the one whose weather takes control of the field.
ITEMS
Most items that are useful in standard singles play are also useful in VGC. Life Orb is just as useful on heavy hitters as ever. Choice items are useful but can carry greater risks if you need to switch and reset it. Focus Sash is arguably more useful since Stealth Rock is unlikely to be in play often.
Leftovers is often not as useful except on the few paricularly defensive/stall Pokemon that are viable in VGC (like Blissey). In many cases, Sitrus Berry and similar items are more useful because they provide a larger and instantaneous HP boost. There are so many phenomenally powerful Pokemon permitted in this year's VGC that several Pokemon would be unable to survive long enough to recover a significant amount of HP with Leftovers.
Resistance berries can be useful for anything that happens to have high defenses but is also Ice/Dragon-weak. Status berries are useful but should be picked carefully. If a Pokemon is much more threatened by sleep than other status moves, it is often best to go with Chesto Berry instead of Lum Berry. White Herb is useful for Draco Meteor users. Iron Ball can be used for Trick Room teams or on auto-weather Pokemon to gain the weather advantage; it can also be utilized as an attack via Fling, removing the speed disadvantage in the process.