I want to remove the decals from my bike
The older the bike, the harder it is to remove the decals. On new bikes they will peel right off. On older bikes a hair dryer or heat gun will help soften the sticker. Be patient and re-apply heat as needed. Most likely it will leave some glue residue. WD-40, Goo Gone, lighter fluid, or turpentine will help remove the residue. As with all solvents, the more aggressive it is, the less time you should leave it in contact with the paint. Wash the area right after you get the sticky stuff off. Follow with a good coat of wax.
About the tank
If you're trying to take the decals off the tank, be aware that they're underneath the paint's clear coat and are not going to come off without a fight. Your options are:
A Goo Gone testimonial
I've had sticky decal residue on my bike ever since I got it a year and a half ago and have tried everything to get it off. Well, apparently ALMOST everything. I had posted a question on how to remove it when I first purchased the bike and received recommendations from gasoline to nail polish remover to WD40 to airplane something or other remover. Nothing worked. Well, I didn't try the airplane something or other remover, but I did try everything else, and they all just made the sticky mess clean.
I'd given up on it until I read a post recently about Goo Gone and I thought to myself, "Self, you HAVE Goo Gone! Why don't you try it?" OK, so I headed over to my bike tonight expecting to be disappointed once again. This stuff WORKS!!! I sprayed it on; it's a gel, so I spread the ooze around and waited for about 10 minutes. I went back with a towel and started wiping. I noticed it had softened the gummy stuff. I wiped some more, then started lightly scraping with my nail behind a towel. WOW! It was coming off! I was so excited! I laid a towel on the garage floor and went to work. 30 minutes later the bane of my existence ever since I'd owned this bike was gone gone gone!!!
If you're having sticker problems that just won't go away, and you've tried everything else, try Goo Gone Xtreme. It is a much more aggressive solvent and softens/dissolves the adhesive way better than other solvents, including regular Goo Gone, mineral spirits, WD-40, and diesel fuel.
A lighter fluid testimonial
Zippo fluid goes for a dollar and change, depending on the size container you pick up. On the several occasions I've used it to remove stuff from vehicles (including months-old sun-baked duct tape leftovers on my fairing a few weeks ago, or the dealer decal goo on my car) it has worked as easily as pouring some on and hand-rubbing it with a shop rag. If the fluid is used up before everything is gone, pour a little bit more on (it works sparingly, but you can soak if you wish). When done, if it's still wet, make a quick wipe across it with a dry shop towel. Evaporates in seconds after that.
A WD-40 testimonial
WD-40 does a great job. I always removed stickers from my car windows with it, but it also works great on plastic. Just be sure to let the decal get well saturated and then it peels right off. Then just wipe with a rag, and it won't leave any residue. I usually apply a good coat of wax after that. Even works on the stock windscreen.