Be patient. Sure, it's hard. If you possessed virtues like patience and foresight, you might not be locked up in the first place.
Keep your mouth shut. More crimes get solved because of stupid criminals than because of smart cops. Don't give anyone a chance to mess up your plan.
Study prison routines. When is staffing the lightest? Who are the least vigilant and capable guards? Which guards can you bribe? Are there periods when the general atmosphere is optimally confused and noisy?
Extend your planning to beyond the confines of the prison itself. You not only need to get out of this building, you need to get into a safe building. Have a hideout and some trustworthy collaborators waiting for you on the outside. Make sure that your henchmen don't harbor any grudges against you and that there isn't a tempting reward for turning you in.
Review your choices regarding hideouts. Another thing about stupid criminals: They're predictable. Stupidity and predictability go together like rats and cheese. Don't go to your girlfriend's or your mother's house.
Have an accomplice get a job with the U.S. Post Office.
Get a job in the prison mail room. Become a model worker.
Make sure your accomplice has a set of clothes for you and perhaps a plane ticket and passport.
Have your accomplice drive the mail delivery truck to the jail.
After the day's mail is unloaded, load yourself in its place and drive out.
If this fails, just make the most of your free meals and wait for massive budget cuts that force the release of hundreds of prisoners.
