GTW,
Speeding up works, but then you arrive over the threshold with a lot more energy, which translates into a longer ground run. For example, if you are approaching at 65 mph and then accelerate to 75 mph, the energy has increased by one- third!
Pilot DAR raises a valid point - the rudder pedal force can become quite high and it is easy to think you have reached the limit. I've always found Cessnas (152, 172, 177 and 182) easy to side slip. I'm lucky that the 182 that I currently fly has the old 40º "barn door" flap setting, so slips are rarely required.
One tip from me is don't do slips while carrying non-pilot passengers - you can scare them - ask me how I know!