Enter: Telling Blow and Crossbow Sniper
Ok. So these two feats might take you some time to get. At level one you can get telling blow (PHBII) which adds your Sneak Attack Damage Dice to any Critical Hit. No limit of range so that’s a big plus!
Crossbow sniper you would have to wait till at least level 3 (you need a BAB as well as Weapon Focus). Make that level 6 as Weapon Focus requires a BAB of +1. So as a rogue you would have to wait. Sorry, no choice there. As a fighter/rogue you could do this around level 3 I would guess by using Bonus Feats from Fighter. Even a single level of fighter may make all the difference.
So. Around level 6 you have 3d6 dice in sneak attack, can add to criticals and, on top of that, thanks to Crossbow Sniper your range is now 60 feet! Thats a bit better, and with telling blow and a Threat Range of 19+ you can get lucky and still kill your target at a distance. At least with 60 feet, if you are on a building for instance, the target or witnesses in the area would never reach you in time for you to make a getaway.
Thought would share this for fun.
Advancement ideas would of course include improved critical, rapid reload, point blank shot and rapid shot (combined with rapid reload and a light crossbow, you can make an additional attack)