Shooting movies on your DSLR drains batteries rapidly, so you’ll need plenty of spares. For the 5D and 7D, each genuine Canon LP-E6 battery runs $100, which can add up quickly. Even the lower-priced T2i uses batteries that are equally expensive: the LP-E8. Nikons aren’t any better on the high end, although the D90 does have cheap batteries available. However, generic imported batteries can be had on eBay for $10. The difference? In Canon’s case, the cheap imports lack the internal computer chip that their genuine counterparts have, which tells you how much battery life remains. The cheap ones, therefore, could potentially run out at the worst time during your shoot: mid-take. Then again, you can get 6 of them for the price of one “real” battery, so they’re an attractive alternative for no-budget filmmakers. It’s important to note that the knockoff batteries won’t work in Canon’s charger; they require their own. The first time I plugged in the off-brand charger that came with my generic batteries, it fried itself in 5 minutes (the seller sent me another); YMMV. Finally, off-brand batteries do not work with the Magic Lantern firmware (as of version 0.1.6). More recently, Series 7 SL-E6 batteries have appeared at B&H, which do contain the appropriate computer chip (they aren’t significantly less expensive than the Canon originals, however).
You can also purchase a battery grip (for the Canon 5D: BG-E6; for the 7D: BG-E7; for Nikons) for double the battery life, although many still photographers purchase a battery grip is for the ergonomic aid it provides while shooting vertically in addition to the extra battery life (you can also use AA batteries with grips). For filmmaking shooting vertically is a moot issue, but considering most support systems require a riser to raise the low-sitting DSLR into a suitable position, the battery grip can also serve such a purpose. Generic battery grips are also available for 1/3 the price of their genuine Canon counterpart (for both Canons and Nikons users report Zeikos makes a quality alternative).
An alternative to using stock batteries is an external powering solution, where a cable can run from your battery compartment to a larger power source; one instance of this comes from battery specialist Anton Bauer:
Switronix has also released an external battery pack for DSLRs, but I haven’t seen any test results to date. Furthermore, Hawk-Woods has created power adapters for the 5D and 7D that offer connection to external batteries: the DC-5D1 and the DC-5D2 (they’re the same unit with different battery connectors). Word on the street is they run around $120.