I started combining images the hard way, cut and paste, non-digitally, then did cut and paste digitally (in paint programs that didn't use layers) Now most decent paint programs have some sort of layering system. Even now, it is important to match your images before combining or else you image will look like the cover of a cheap supermarket tabloid magazine.

TIP #1. Look at where the light source is, in the images your combining, look which way the shadows fall. If the images don't match, try and make it so by adding shadows or adding light. Sometimes, it's better to replace an image that isn't working out well. I'll spend hours and sometimes days looking for an image or images that will work together. At this moment I have images sitting on my desktop just waiting for that one special element. Sometimes, I'm not sure what that element will be, but I'll know it when I see it.

TIP #2 Every image has a texture, some are clear and crisp, other may be grainy and blurred etc. Try and match textures.
One method is to add grain to either of both of the images your combining. If your adding grain, it's usually one of the last steps in a manipulation. It's not may favorite method but I have used it. The large view of my "Tonights Date [link] " shows how grain filter is used to help blend 5 different elements. It looked pretty good before the "graining," but the grain also added to the feel of the piece.

TIP #3 Try and match saturation and contrast levels before combining images. Important if you want to achieve that non manipulated look.

TIP #4 Work on the manipulation in 2 to 3 times larger the resolution than your intended output. Example: You plan your image to be 800 x 600 when you present it, scale up the elements to 1600 x 1200, combine them and scale them back to 800 x 600. Down scaling an image will hide imperfections nicely.
Ever notice when you click on a great looking thumbnail here, and when the larger view loads, it's less than you expected.
That's a prime example of how down scaling can improve an image.

Oddly enough, the opposite happens with thumbs too. A sad thing when a wonderful piece of art is hidden away because of a bad thumbnail on DA. I imagine allot of pencil and dark artists must realize this. Thumbs of their works quite often hide too much detail.

TIP #5 Watch your image edges, too hard an edge makes your image combination look obvious. My preferred method of "blending the edges" is manually using a small fuzzy eraser brush carefully around the edges.

TIP #6 Don't flatten your images layers until your positive you've done all you need to do within the layers.

TIP #7 The UNDO and HISTORY are your friend!