DH and I use limo and private car services throughout the country for business trips and vacations. The general rule is you tip the driver separately for each trip unless you are absolutely certain that you will have the same driver both ways. The size of the tip is generally based upon his/her promptness, how courteous he is, the number of bags and other items he has to handle, the number of people in the party and if he does you any special favors, such as the grocery stop on the way to WDW. If the service is above average, round up or tip more. Generally, the comments I have seen on these boards are in the $10-15 range each way.