Oh no no, it took longer than a day, it goes all the way back to Life Force on NES. After you get the hang of moving the ship around and shooting things, every game that comes afterward is just a variation on what you already know and the focus is on tweaks you can make to get good at that particular game.
If you really want to get better, I can offer a few general tips. The most important is probably learning the enemy patterns. The way I go about that is starting from level 1 after each game over, and keep that up until I can beat the game. It's kind of inefficient in this game since there's a level select, but trust me, you'll be really good at the earlier levels by the time you're done. Also, keep your gaze focused around your ship rather than the mass of enemies when there's a lot of action going on. You'll know what enemies are spawning and what their movement patterns are soon enough if you restart from scratch after a game over, so you'll be able to tell what that little blob at the edge of the screen is without focusing on it just by the way it moves around on the screen.
As for Resogun-specific stuff: After each phase and the start of a new level you have a grace period of 5 seconds or so where your multiplier timer is stopped as long as you don't kill anything or pick up/drop off a human. Use that to your advantage in areas where it can be tricky to keep your multiplier going (beginning of level 3 is the only one I can remember atm) and let some enemies spawn and get close before you start shooting.
Great places to use overdrive: The BEST place is on the level 3 boss. With a 15x multiplier on veteran, using overdrive on the big gray cube it spits out is worth ~15 million points or so. If your multiplier isn't maxed out, you can just hang out for a while and build your multiplier up shooting grey cubes, then hammer on the boss until he spits out another and then OD. I pretty much only use OD as a point generator. It really does net a lot of points if you have a high multiplier and get a lot of enemies to shoot, so I use it that way instead of as a backup boost button.
Use only as much boost as you need! This is especially true with Phobos, since it is a sitting duck for a long time if you get caught up in trying to save humans on the opposite side of the ring or blowing through those long formations of ground turrets. Boost is your primary method of escaping bad situations in the later levels.
Bombs: LOL I dunno. I occasionally use them to keep multis going, but more often hoard them and blow them all on the later levels. Sometimes I even game over with one or two left in reserve like I can take them with me to the afterlife or something. I'm bad with bombs.
Boss stuff: On the level 2 boss, get right down on top of each panel between the turrets to take them out, then skirt along the perimeter between turret shots to move to the next one. It's way easier than hanging on the outside as the boss plays keep away with the remaining panels. Once he changes forms, get on the top or bottom of the screen, run away, and dodge bullets by changing speeds. When he exposes the green spots you can take the first one out and boost to the other side and get the other before he goes back into spaz mode with Phobos.
Oh, and RELAX! Every enemy in the game has a pattern, while you can move anywhere you want at any time. It's kind of like a puzzle in that way, and the goal is getting yourself into positions that you can escape from and taking things out in order of priority. I'm not fast enough to play twitch games, and thankfully Resogun is NOT a twitchy super-fast reflexes MLG4LYFE kind of game, at least on veteran and below. It's kind of this perfect mix of tense and exciting but not frustrating. You can get really good at it just by treating each game like a learning experience.
There's lots of little stuff that really doesn't have any right answer (when to save/not save humans, which ship is the best, etc), but hopefully some other folks will hop on and share some of their insights (I'm looking at you, JoeFenix!).
