Enter WeaponInfo.xml with Notepad. ..\Rockstar Games\Grand Theft Auto IV\common\data
Find RocketLauncher-part in this file and look after clipsize for the launcher.
Change value from 1 to for example 10. Save!
If you fire a rocketlauncher, Niko won't put in another rocket to launch, he just keeps firing rockets. But you must hold down fire button to fire more than one rocket. If you release the button, he will reload.
That's my tip for the day!