As a terrorist, play on a server where you can pick up weapons with no ammo. When you pick one up when you are terrorist and it has some ammo, keep the gun. Try not to use any ammo. When the round ends, drop your pistol and keep you main gun (usually an SMG like the MP5/40 or the UMP 77). You will get your pistol back on the next round, but it will be equipped with more ammo. If you check your main gun, you will notice that it to will be equipped with more ammo.