For dining rooms, the easiest (and fastest) way to decorate is to get a set. You know everything is going to work together, and in one purchase you've got the room almost complete. But if that idea sounds kind of blah, try collecting a group of mismatched chairs to add some interest. There are are just a few guidelines to follow when doing so.
First, try and keep at least one element in common for the whole bunch. For example, all the chairs in the top image are distressed wood with a white cushion. The set in the second image are all a similar style (high-back wood) and each is painted. The colors may be different, but there's a uniformity to the look, nonetheless. You could also do all plastic chairs, all metal, or several shades of bright colors - there are plenty of options.
Then, make sure to measure the desired height you'd like the for the seat of the chair to be. It's okay to have some leeway here, but be careful that it doesn't sit too low to the table, or so high that you're legs are cramped underneath. The key is to maintain a comfortable position for each seat.
Once you've figured out these two steps, you'll have narrowed down your choices quite a bit. It should then be easier to put together a set that suits you and your space.
Top image: Joanna Henderson
Bottom image: On Bradstreet