First of all, lets just get one thing out of the way. They are Zombies, and you are still human. Ultimately this leads me to my list of general tips for Resident Evil 5:
1. Press A and hold Back on the control stick. This will wheel you around 180 degrees giving you an excellent opportunity to RUN AWAY! This isn't your average run-n-gun shooter. This is a stop and pop shooter, therefore you will need to keep putting distance between you and the hordes of zombies if you hope to survive.
2. Staying near your co-op companion is essential (especially in the harder difficulties).
3. Make those shots count, because lets face it, there are more zombies than there are bullets.
4. Your knife is as useful as a can opener. Use it merely to open crates or if you need to finish a guy who already has a shotgun casing in his chest, and a bullet in his brain.
5. Sharing is caring. When playing with another human, try to use weapons that use different ammos, if you cannot reach a consensus, be kind about ammo. A friend without ammo is as useful as his knife, and if you read tip # 4 you already know that he is the weakest link.
6. The magnum is only a useless pistol until you get unlimited ammo, until then you should expect to be carrying around an empty pistol for most of the round.
7. Play with a buddy! The AI is an ammo waster, and isn't all that bright. The AI wont get in the way, but they sure wont be the MVP of the match either. Play online, or find a real human to split that screen.
If you are looking for a review of Resident Evil 5 to see what you're in for, look no further than here.