1. Gear up.
No class in all of TOR is more gear dependent than a JK. The right types of gear is just as important as having new updated gear all the time. If youre picking boxes instead of commendations hoping for an RNG roll for new gear, youre doing it wrong. If youre damage, go for strength and endurance. Power, crit, and accuracy are also great sub categories to go after. As a tank, defense to cap is good, but remember, too much defense and not enough strength gimps your damage. Make up for it any way you can with stims and the like.
2. Gear up your companion.
Companions can make up anywhere from 33-66% of your DPS. Go try and solo an elite (gold star) mob without one at level and see how it goes. If you did it without cooldowns, youre a God amongst men. If you did it at all and blew all your cooldowns, great kid, dont get cocky. Gear them to their strengths as well. The gear that they are wearing when you get them is a great indication of what you should be outfitting them with stat wise. Set your sights on that and go for it. Youre only hurting yourself if you dont.
3. Use your companions strength.
I got to 50 using Kira Carsen as my companion for 95% of the game. I was also DPS spec. For those of you not using Doc as your healing companion, this advice is true for you: start your fights with the companion. The game gives you the ability to choose what abilities your companion does for a reason - this also means the companion can start the fight and get initial threat from the group while you mop up. If theres a silver or gold enemy in there, send your companion at them first. If theres normal non silver or gold mobs in the pack, charge in right after and take them out while your companion tanks the big guy. Then take them out after. When your companion is at about 20% health, go ahead and taunt the enemy so it attacks you. A companion at 20% life does 100% DPS while a companion at 0% life does 0% DPS.
Remember, as Yoda says, "Always two there are. No more. No less.
4. Prioritize your targets.
If its a pack of 6 regular enemies, send your companion in first to some targets on the left while you go after some targets on the right. Splitting your DPS for this is a good strategy as it also splits up the damage taken. 1 gold elite with 3 regular mobs? Send your companion after the elite while you kill the regular mobs. 2 silver? Send in your companion to 1 silver and make them use a defensive cooldown and focus fire down one of them. Try and use your stuns and knockdown abilities on the other to mitigate damage until the first is dead and then move to the second. Going after the hard target and leaving up the weak ones is a recipe for disaster.
5. Use your defensive cooldowns.
The JK is given LOTS of defensive abilities between stuns, knock downs, damage reductions, temporary health boosts, and slows. Dont be ashamed to use them. think of your companion as a second life bar as well - make sure you use it, but make sure it never hits zero as well. Also, use medpacks. They are a big deal and can save your life. If your companion is at 20% health and you taunt the mob to you and you are now at 40% health while the mob is at 60%, use a medpack. Theres plenty more out there and you can buy them from a vendor.
6. Interrupt the enemy casting abilities.
There are plenty of bosses in Flashpoints that you wont be able to interrupt abilities for and plenty that you will have to, but just about every boss you fight for story related purposes outside of flashpoints has an interruptible cast. These can range anywhere from a high damaging ability to a big heal. Learning when to watch for them and when to interrupt them is just as vital as breathing. If you are at 40% and the mob is at 40%, letting that heal get off so its 80% for them and 40% for you will mean all the difference. Dont let it happen. You have many tricks to stop it from happening so use them.
7. Learn to play.
This was one I debated putting on here, but really, it comes down to you reading the tooltips and understanding what each ability does. Same goes for when you throw those points into talent trees for your advanced class. Use the infinite resources of the Internet at your disposal (really, all you need is torhead.com) and spec out a character that will maximize damage and survivability.