A few general things to keep in mind while editing photos...those programs are memory hogs. Every time you do something to an image, it makes a snapshot of it for the "history" (so you can "undo" it later). If you've got your history states set to a high number, you're sucking up more memory.
Have only one image (or as few as needed if combining them). The more you have open, the more memory the program sucks up. If you're done with a 2nd (or other multiple images), close them.
Some programs will allow you to tweak the amount of RAM that the program can use (via "options" or "preferences"), and usually 85% is a good number - high enough not to take away from other programs, and low enough to do what the program needs.
If you see the program starting to lag, save your image and restart the program. If it still lags, restart the computer, as that will free up RAM being held by programs.
Limit the amount of programs that start when Windows starts. The more programs you have running, the less memory you have for other things. Not to mention that some programs "run in the background" which slows the processor down.
Don't make your image larger than it needs to be...in more ways than one. If you're making an avatar for a website, and it's only going to be 100x100 pixels, then start it out at that size, rather than making it huge and then shrinking it down. Also, keep your image's PPI (Pixels Per Inch) at a decent size. Anything for the internet will look fine at 72 PPI, but no larger than 100 PPI. Images that you'll be printing should not be higher than 300 PPI (as that's usually the max for high end printers). If your printer is only 200 PPI max, then match your image PPI to your printers PPI. The higher the PPI, the larger the image size is and the more memory it sucks up.
These are just some off the top of my head...hope it helps!