Here are a few suggestions that have been made to me by others who have been successful:
1. Visit some galleries and find two or three that seem to show work that is of the same general calibre and type as your own. Visiting websites can help.
2. Find out as much as you can about the tastes and prejudices of the gallery owners. See if their websites say anything about their submission preferences.
3. Prepare a targeted portfolio of your best work. As has been mentioned elsewhere, gallery owners seem to prefer themes to variety, so if you have 20 really good images of old trashcans (for example), it is better to show all 20 of them than to show one trashcan, one baby picture, one beautiful landscape, etc.
4. Some gallery owners want you to leave a CD or a portfolio with them and they'll call you when they have looked at it. Others want you to make an appointment to sit down with them to look over your work. Some gallery owners want to see finished prints and won't look at anything digital, while others prefer jpegs. Again, learning what you can about their prejudices before you go in can make a difference.
5. Your work should be easy to look at. For example, twenty to thirty prints of letter/A4 size in a nice portfolio case, plus one or two examples in the size you prefer to display is easy to evaluate.
6. Titles or no titles? "Giclee" or "Ink jet"? These preferences can often be determined by looking at the current exhibit at the gallery.
7. Don't push technical information unless asked for it. If you are asked, have it handy.
8. If possible, talk to someone who is already represented by that galley.
I hope this helps a little. And I'll let you know if and when I find the gallery of my dreams. You do the same, please.