Best way is to boot up a lan server and screw around yourself. Gets you a nice handle on game physics, mechanics, and what you're capable of.
I suggest this for anyone who wants to become a better alien. Marines are pretty straightforward for anyone that's played an FPS, but for the trickier alien ability combos, like a good skulk leap-bite or a fade blinking, it's much better to practice on an empty map to your heart's content. Just turn sv_cheats 1 on in console and type spawnhive a few times and givepoints a few times to add res. Then fade away and practice with any abilities you want. The best fades and skulks are not born that way, they're FORGED by hours of practice. Practice I'd rather have you doing when we don't need that last fade to save the team...