What they said: more light!
I am picking up a 400W HPS light for $40 off Craigslist tomorrow. Got a 400W CMH bulb to use instead, they're cooler and have a wider spectrum.
Alternatively (when one can't find steals of deals on CL), Amazon has 2-foot, 8-tube T5 lights for around $120. You could do probably 4 biggish plants under one of them. The tubes do wear out, however, and replacement T5s are.. not the cheapest.
But your cheapest bet is probably CFL for a quick-n-dirty grow. These might come in handy if you decide to go that route:
http://www.ebay.ca/sch/i.html?_sacat=0&_from=R40&_nkw=bulb+socket+flexible&_sop=15 Great for daily adjustments, and keeping the bulbs within 2" of leaves.