If you have their email address (and you know they check it routinely) you can drop them a line saying how you understand they are busy, but you wanted to remind them about the letter of recommendation they agreed to write. Also tell them if there are deadlines you are trying to meet, and ask them to let you know if they are going to be unable to meet them (hopefully you have back up options if this is the case). Give that at least a week for a response, then you can think about talking to them in person. You can track them down where they work, or make an appointment. Ask them if there is any additional information they need to complete your letter, mention any deadlines, and again give them the opportunity to back out. If that still doesn't work, there isn't a lot you can do that won't come across as pushy. You should probably just cut your losses and seek other letters at that point.