I have a Garmin 60csx. Using their MapSource software, I can preplan my route while sitting in front of my computer. You must prevent the GPS from automatically recalculating when you're off route, else you'll lose your preplanned route. The recalculation can be set to prompt you. It will sound an alarm letting you know you've deviated from your plan and give you the option of recalculating (which you likely don't want to do).
I believe there are newer units available that have these functions as well.