While the uplula is very good, I've found that loading the magazines on a table or even with the base against your leg gives you enough of a steady platform to put the cartridges into the magazine very easily.
Another trick is to tap the magazine (or "bang it") after most of the rounds are loaded. This lines the cartridges up so there are no hang ups which make loading more difficult. Sort of like the Vietnam era GI's banging M16 mags against their helmets before inserting them into their guns.
Check out 0:46 of this. At least the rest of the video is fun to watch.