Disclaimer: I am not responsible. Period.
Ever wanted to score higher in Kitten Cannon? How about owning at Defend the Castle from the very first level? Some jumps in N-Game too hard for you? Maybe you're ticked off by that guy who got several billion in Push the Button. Whatever the motive, you're here for one reason, you want to hack flash games.
All flash games are written in a code called Actionscript. It'll help you if you know it, but the code is straightforward enough to where even a n00b could do this with almost any game they wanted.
So let us begin. In order to hack a flash game you're going to need a few things:
1. Flash Game
2. Flash Disassembler
3. Umm, you don't actually need much to do this!
So on to the stealing.
1. Getting the program.
A wide number of utilities exist which allow you to extract the guts from a flash game. One of the simplest and most versatile, the one I'll be using, can be found here. Download flasm.zip and unzip it to any directory. Also, flasm does not have the capability to extract the source from projectors (.exe) flash files. If you're dead set on a projector game I recommend sothink, but I that I won't be discussing projectors any further.
2. Getting the game.
When choosing a target game there are a few things to keep in mind. Some games keeps files separately, such as their sounds, and they will refuse to play after you download them. Newgrounds.com is a great place to pirate your games from, as they require authors to upload everything as one file, and most people do not secure their script. This guy looks like a good target. Unfortunately, the high-score server for this game has already been disabled, so there's no real rewards to reap, but it's a good learning exercise.

The process of downloading a flash game is simple. Right-click on the web on which the game is located. DO NOT RIGHT-CLICK ON THE GAME ITSELF, cause that gets you nowhere. If you're using Firefox, go to Page Info. If you're using IE, get firefox.

On the page info, click the media tab. Find an embed that jumps out at you. On newgrounds, it'll be the first embed listed. The file name should end in a .swf. Save the game to your flasm16win directory.

3. Editing the Game
Time to extract the source. Start > Run > type in "CMD" to bring up the command prompt. You need to get in your flasm directory so type "CD C:\Your Flasm Folder\".

Now you need to extract the source code. Type "flasm -d yourgame.swf > yourgame.txt".

This will create a text file in your flasm folder. Start looking through the text file. Try to find a variable or number that looks suspicious. For instance, if you're playing a game where you start with 330 gold, or every time you kill a guy you get 12 points, then it might be a good idea to do a CRTL+F search for 330 or 12. SCORE, GOLD, LIVES, and GUYS are all pretty big hitters. In Jack Russell you get ten points every time you pick up a bone. Scan through a few searches for SCORE and there it is:

I'll just change that to umm... 3,000!
Now it's time to put the code back in the game. To do this type "flasm -a yourtxtfile.txt" and everything should be put into place.

Let's try our little edit out, shall we? As you can see, my Jack Russell is doing pretty well for himself.

Things to keep in mind:
-Rough numbers are more believable on a high scores list, should you wish to stay on one.
-If you go too high above the other people, you're likely to be taken down.
-If you screw anything up, I am not responsible.