How to get rid of a decal on the rear window?

Acetone and isopropyl rubbing alcohol (90% and higher) are very effective at dissolving glue without affecting glass. It will affect any other plastic layer on the window so you should check for any anti-UV screen or other.

I suggest a combination of Steve Matthews' answer (remove the plastic with heat) and afterwards get the eventual glue residue off with Acetone and a rag.

Beware Acetone is a dangerous chemical : wear appropriate PSE (googles, gloves).

On glass, heat would probably work well as mentioned, but I've worked at a car dealership for years, and always removed stickers and window paint with a simple straight razor blade and windex. The windex is really just for lubrication - spray a bunch around the sticker, and scrape it off using the razor blade at an angle. Any remaining residue can be cleaned up with more windex and fine steel wool (the finest you can find). Detailer's trick ;)

Word of caution however, DO NOT DO THIS if it is on the inside of the rear window and you have defrost lines, OR if you have aftermarket tinted windows, as razor blades and steel wool can easily ruin both.

If either of these are the case, you are in for a bit more of a hassle, and I would suggest just sticking with just heat and windex or some sort of goof-off and a rag to work at it.

Sticker remover or sticker adhesive sprays and manual scraping work a treat.

I'd use a plastic scraper or credit card to avoid damaging rear defroster lines.

For anything that's just glass, I would follow @James' answer regarding a razor blade and any glass cleaner; it's definitely the best tactic.

For the inside of the rear window where you have the defroster lines running, I've had the best luck with the aerosol "professional strength" version of Goof Off (or likely any other xylene/heavy solvent based cleaner). I used this along with a liberal amount of shop towels to remove the entirety of the old cheap rear tint residue on my rear windshield.

Heat is the first answer, but I've found that even with heat, I've had residue left over. Goof Off didn't attack the defroster lines, but worked wonders against the leftover adhesive residue.

Stickers are constructed of a huge variety of adhesive - some with a greater affinity for the glass, some with a tendency to stay on the underside of the sticker. The first step should always be to lift a corner (with thumbnail or properly mounted safety razor), grip the corner and pull; begin with a slow pull and note which surface the adhesive is sticking to. If it is sticking to the underside of the sticker, it will avoid having to follow-up with a solvent to remove it from the glass. If the slow pull leaves the adhesive on the glass, try pulling more quickly; the adhesive may then stay on the sticker. However, a faster pull may tend to tear or shred the sticker, complicating the removal. Occasionally, razoring the line of detachment as it progresses can be the best approach. Caution: Razors can inflict very serious wounds!

The glue from stickers of most kinds is easily removed with WD40, the same stuff used to stop squeaks and lubricate things.

It will mess up the finish on wooden furniture, so this is not a general solution, but perfect for glass or metal, like OP's issue or when a used computer has a sticker from the reseller.

There is a citrus based cleaner called GooGone that also works great for removing many adhesives. And it smells nice when you're done. I use it frequently to remove price tag sticker residue after removing as much of the sticker first by peeling or scraping. A google search shows that a lot of places carry it.

A straight razor blade, Goo Gone, and warm soapy water. Works like a charm.

1. Using the blade, gently scrape away the sticker material.
2. Use Goo Gone according to the directions to remove any residue.
3. Wash away any leftover Goo Gone with warm soapy water.

Edit 1: Hopefully it goes without saying, but this works on glass only. Don't try it with painted surfaces.

Naphtha, available at any hardware store in the paint section, works beautifully to remove sticker residue. It is commonly used in furniture refinishing and luthiery for applications similar to that described.

It will dissolve and lift any sticker adhesive I've encountered off glass, metal, plastic, ceramic, and even lacquer without damage to the underlying finish. It won't dissolve plastic like acetone, and doesn't require any razor blades or scouring. Just saturate the sticker with naphtha, and then wipe away the residue with a shop towel.

It also works quickly on those very difficult-to-remove notices placed on your window after your car is booted or towed, again with no knife or steel wool required.

Peanut Butter

1. peel off sticker's plastic coating, if there is one
2. smear some peanut butter over the sticker, leave it until the sticker gets soft and comes up easily. 20min - 3hrs
3. wipe it off, the sticker and glue should come off easily. clean residue with soap and water

Why does this work? Oil breaks down that kind of sticker glue. But if you rubbed oil on it it would run down the windshield, and not sit there long enough to work. The peanut butter stays put, allowing its oils to work.