1) I do Kashyyyk first, to pick up Jolee. That and its the easiest one IMO
2) Then Tatooine, to complete just about everybodys´ side quest (Mission, Canderous, Bastila) O.K so theres only three, but you get my point.
3) Then Manaan- save my favorite for last.
4) Korriban - its the hardest one plus it just seems to fit in the storyline like that that and Bastila is already gone soit makes no difference if she can set foot on Korriban anyway.
Ascendant_Justice gave a good explaination, though Kashyyyk and Tatooine can be switched without much of an issue really. True, Tatooine, Kashyyyk, Manaan, Korriban, is the order Basty lists them in...I just prefer to pick up Jolee before HK. I would argue that Kashyyyk is easier than Tatooine despite what anybody says though...
Really as long as you don´t do Korriban first, you´ll be fine.
Its very hard if you do it first, and makes it impossible to complete Carth´s sidequest.