Having had some success with these methods and having worked for a sticker manufacturer here are some tips or observations that may help and inform you.
Decals can be removed using this method especially if the paint is strong. Simply use regular Scotch tape and press it over the decal. Rub it on good then rip it off and you will take most of the decal with it. Repeat this process till the decal is gone. You can vary the strength of the tape. Use more aggressive tape for harder decals and less aggressive for easier ones. I find that regular packing tape really does well. Also vary the way you remove the tape, slower for delicate areas and quicker for very strong painted areas.
Now for stickers and this is a little trickier. There are two basic types of stickers. Ones that are removable and those that are not intended to be removed. We will discuss the later first. Stickers that are not intended to be removed are called chip away stickers. They are used for bicycle licenses in many cities. They are sometimes reflective in nature and are designed not to be peeled so you cannot re-use the license. They literally have to be chipped away a little at a time. Use of heat will help but patience and a non metal scraper with some mild solvent is your best bet.
The removable types are usually made of mylar. Mylar is a plastic film that is real strong unless you cut or notch it. Think of the potato chip bag that you cannot open no matter how hard you try. You notch the bag with your teeth and it tears with no effort at all. You can use this to your advantage. Most manufacturer bicycle stickers are mylar. If you use mild heat and just get an edge up without nicking, notching or cutting it you can very slowly peel the entire sticker off with one continuous pull. Of course if the sticker is notched already or scratched up pretty bad then you will have to do it a piece at a time but with patience it will be a lot easier and you will not have to deal with those nasty solvents.