Replying to Batteries for nonhd GoPro

Sorry to make another thread about these but I haven't seen this posted yet.
What types of batteries are you guys using? I can't afford lithium batteries every time i use my cam so I bought 6 rechargeable nimh batteries and a charger. after fully charging them I only got 8 minutes of filming before they died. and it was inside where its warm. Any tips on getting the most out of your battery life? or should I just suck it up and buy lithium every time?