I use Eneloop batteries. They were the first on the market with batteries that retain charge inbetween uses so you can leave the batteries in the flash for months and they won't drain down whilst kept there. AT the time I bought them Sanyo were the only ones making them - now most brands have their own equivalents so you've more market choice.
Also a BIG thing is a good charging unit. Batteries in flash are recycled fast so they gain a lot of wear and tear so you've got to have good charging unit that lets you slow charge the batteries (think overnight). If you use the quick half hour or one hour charging units you'll kill the lifespan and capacity of your batteries very quickly.
Maha/Powerrex make a very solid series of charging units; sure they cost a fair bit, but they pay for it in quality and in how they keep your batteries in good condition. You save money because when you're using smart batteries that retain charge and a good slow recharging unit you don't have to suddenly put down £5 here and there because you were out and your batteries died and you've got to go to the corner shop to pick up some more quickly to keep shooting.
It also means that your flash is ready when you are; you don't have to worry that "Oh I'll get a handful and then its dead".