are the brakes able to cause the wheel to skid?
if yes, then you can't get better braking than that.
to do so, you'd have to change the tire itself. or change the surface the tire is skidding on (meaning, ride somewhere with better road/path surface--not usually an option).
if no, then you could try changing the pads, or the braking surface.
pads are easy. there are lots of pads out there, with different compounds.
the ones i've had the best luck with are by koolstop, the salmon pads if for wet conditions. you have to get the right pad style for your particular brake arm and rim surface, but the koolstop website should have enough info to help you pick the right pad style. if the salmon ones don't work you could try other compounds.
if none of the pads make a difference, then the problem could be in the braking surface itself. if the surface is excessively smooth, like steel chromed rims, or painted or anodized rims, the pads can't grab it very well.
if the levers simply have a lot of pull before you get significant braking, your housings or cables might be a problem. if they're bad enough, changing those to just about anything new and setting them up correctly would help, but there are some "really good" housings and cables like jagwire that are designed for very low friction and compression, so more of your lever action results in braking, without haivng to first compress the housing and then stretch the cable, and without letting the brakes "stick on" that wears the pads out.
adjusting the pads for evenness and getting them as close as possible to the rim with the lever unpulled, will help you get more braking sooner.
if you end up having to go disc (which i doubt), the avid bb7 cable-operated mechanical brake with 200 (203)mm rotor is what i use on the front of my sb cruiser trike, and that will skid the wheel (and give fine control up to that point) from 20mph; the trike is about 500lbs with me on it, and it still works to skid the wheel even with another 300lbs+ in the cargo area. if i had better weigth distribution, it would brake at higher levels before skidding...at some point i'd still hit the traction limit of the front tire, and skidding would still happen.
