Imo first you need a comfortable crosshair.
Try to shoot when you really think it's gonna hit; patience is key. It's also important to get the feel of the rocket's speed. Try to visualize trajectories, since it's not a hitscan weapon. With enough practice, it will be second nature.
For a nice bounce, you have to aim slightly below the legs or at the "heel" area. Don't forget that many times, hitting a good spot on the wall / ceiling / floor can produce nice splash damage. With properly placed rockets you can force your enemy to be in very unfavorable positions - you should use these instances to your advantage; he's / she's almost bare naked when he / she bounces around (in a directed manner) helplessly.
Learn the spots on every map, where can you expect nice spam damage. When you spam, don't just press +attack in every direction, make it so that every shot have a purpose.
A *.cfg that provides sufficient depth perception can also help a bit so as cg_playerLean 1 since it will be easier to visualize the forementioned trajectories.
Learn and think about what are those situations and ranges where you have an advantage with rl - use the weapon, when it's appropriate. Let it be a fine tool of destruction, not the only gun of a +forward rocket monkey. : D
What I did back then when I didn't have internetz is just loaded up dm17 with a nightmare bot (Anarki / Sorlag most of the time) and tried to hit as many air- and long range rockets as possible. Your rl aim will improve with time, just don't expect instant success. Practice, practice, practice.