*If there are a lot of traffic coming through petrol station, or in general, don't waste time going to speek to drivers in rest area, they will pass you at some point.
*Try asking drivers if you are in good place for heading that direction, for example: Strazdas007 was stuck on Maidstone services while trying to get to the North, as all drivers where going South, or just around the corner, so he just went back to Dover port. If you are in bad service station for your direction, don't hesitate going back to last good spot, or possibly good spot.
*Try to look up service stations on maps or gps.
*Go for the popular ones if you have a choice, like: Starbucks over Costa, as it is with petrol stations - Shell over BP.
Your driver will also get a cabin with his ticket with four bunkbeds in it, so you will depending on wether your driver wants you in the cabin or not, get a night on real sheets. (Cheapest alternative I have been able to come across if you are still to do it in any legal way. Credit to SamanthaofTarth on the westeros internet forum for this initial info.)
(If you decide to pay, consider getting a Megabus
für 25 Pound from London to Rosslare, ferry included, as this is already cheaper then the foot passenger ticket for the ferry only. Alternatively, there sometimes are decent Rail & Sail offers from London to Dublin, via the Holyhead Ferry.)