Make a work schedule. Say you'll work on Commissions Monday through Thursday, and then Friday through Sunday, you work on personal stuff! :)

Of course, it's just an example.

ive done that before, but i always get mixed and lose track

Heh, I've had the same problem :'D
But recently I've been using working on personal art as sort of reward for getting certain amounts of owed work done

Let's say you've gotten the commission sketched out and inked, you could then switch to working on a step for a personal piece (like if it still needs coloring or shading)
After you completed a step or two on your personal piece, you can come back to the commission and do a step or two on that as well

I've found this to be a nice way to balance commission and personal work since more gets done quicker, and you don't end up getting tired of working on the same piece for several hours straight ~

If i can't seem to stic to a scheduel, i'll give this a try!