Alex (
ales) has been banging his head about this for months.
Some sets require an topcoat seal, some don't, and I'm sorry to say I don't know which is which.
My hobby store has some kind of all-in-one DIY decal set, but I haven't looked at it too closely yet.
Are your decal sheets clear or white? What colors do you need the logos to be? And what color is the car?
Edit: Here's Alex's thread:
http://www.automotiveforums.com/vbulletin/t5188.html