Lower Draw Distance.
Turn ON FPS Limiter.
Use low-rez effects and water textures.
Avoid highly player-populated and mapped areas.
When in-game, press ALT + ENTER, in windowed mode there will be less lag.
Turn off Google Chrome and Skype.
Here is the deal, this is a SA-MP problem, since 0.3x. Processing objects and players works differently in 0.3x and it causes lag. There is not much you can do but what I said above. I feel your pain, I'm having same troubles.