Drink a LOT of water. It will keep you hydrated and send you to the bathroom quite frequently too. I don't know how much I'm going to be peeing in my 21st week but I'm already going every 15 minutes to an hour. If I'm 'trying' to stay very hydrated its more often.
i have blood clotting issues too and am about to embark on a 10 hour road trip to PA....i just talked to my doc about it and here is what she advised:
drink plenty of fluids (water) and empty your bladder frequently
move your feet around and change position of your legs
get up and walk every 2 hours at least
be aware of what the symptoms are of a blood clot
make sure you know the local hospital where you are travelling too
try to keep feet elevated
hope this helps :)