I keep reading contradictory information regarding whether you should use a conditoner on the day you go to the salon to have it dyed or not.  Some say you should because it will give a more even result where as other sources say you shouldn't condition so the dye can penetrate deeper and some sites say you shouldn't even conditoner for 14 days before having hair dyed   Well, the last advice is too late for me because I'm having streaks done next Thursday and I condition every day.

The type of conditioner you use could be the biggest factor.  Conditioners that have silicone based ingredients (generally anything that ends in -cone), will probably prevent the dye from taking as well as it could.  I'd suggest doing a vinegar rinse the last time you wash before you have the coloring.  This will help remove any product buildup and give the coloring the best chance to work with your hair.

I also agree with Sakina - checking with your colorist for their recommendation would be a great idea.