how do i get one off?
Dunno, mine fall off! lol
Use a hairdryer to soften the glue, then peel back slowly? 
I dont know, so maybe listen to others first.
Then wash the remaining glue off.
ditto hairdryer, but then you have the dreaded adhesive…
the guys were really helpful on here when i had the same problem: 
http://londonbikers.com/forums/shwmessage.aspx?ForumID=37&MessageID=113404#bm115732 petrol seemed to do the trick for me WD40 just didnt seem strong enough.
Nitromors?
(CM runs for cover…)
Most bugsprays will get the glue off. Maybe that is how bug spray works on bugs… 
Yep a small bit of petrol on a clean rag worked for me and clean it off after of course
Zippo lighter fluid works wonders on glues - use it all the time to take those annoying sticky labels off
Although in good nick I wanted to reposition mine so peeled the bugger off. It left all the adhesive behind so I basically softened it up with some white spirit and spent at least an hour rubbing it off with a combination of rag dipped in spirit and my bare hands 
Made sure tank was clean before fixing new one…dont fancy doing that too often
Well happy with the outcome though