Pick simple locations. A neighborhood book store is less likely to give you problems then, say, the FBI building. Find locations that are smaller, simpler, friendly, and more willing to help you out.
Develop characters to their fullest. In a low-budget movie you are not going to get Nicolas Cage or Brad Pitt, so the characters will need to attract audiences. In most big-budget movies it is the actor that draws people, and you need the character to do so here.
Plots should be based around the characters as opposed to the action or central conflict. The best low-budget movies tend to focus on the happenings of a few friends across a few simple locations, either reliving past experiences or developing new ones together.
No explosions or special effects. This will cost your entire budget. Green screen is okay, but nothing supremely fancy.
Make sure the timeline of the film stays small. It will be much easier to film a low-budget movie taking place over a day or a week than one taking place over three years.