I suggest looking at the bigger picture. Electric chair works best for me when I look at it as a piece of the puzzle that is half guard bottom. My goal in half guard is to get the leg hooked with one arm and I have my other (higher) arm ready to reach around their back. then once I am ready, I wrap my arm around their back and I go for Dogfight. this has they same principle as pressing into them except you are aiming to get somewhere and if they don't fight back then you will get there.
Then I go for the electric chair and the thing I've really found important, is the timing of swinging my legs. I found that if I twist with my upper body first and then follow through the legs, I get the best results. (I think everyone should figure out timing for themselves though)
If electric chair doesn't work, and you just can't budge them, you can start stomping and working from either butterfly guard or homie control/prison guard.
I see half guard as a transitional guard. It is a very very active guard, where you cannot rest, it has to be prodding and pushing and pulling. make physics work for you rather than against you. remember the three bases rule. switch things up and practice not just each technique by itself but as a whole and you will really start seeing results.
Of course Brent is right in saying there are other sweeps to go for, like old school etc, I prefer the mixture of dogfight electric and stomp because they seem to attack their base from different directions more than others. I'm sure as I get better, I'll add in more parts.
I hope this helps.