It's not JUST PC muscles to work on. There is a second part of ejaculation control that must be worked on, but is generally neglected/ignored.
See, most guys do kegals fine, but they do them in a manner that elevates the testicles. That's "bad form". Like any exercise, form matters. The proper kegal form is to contract the muscle that cuts off urine flow WITHOUT clenching the muscles that elevate the testicles. You'll need to watch what's going on down there to get a feeling of which muscles are doing what. Once you've isolated only the muscle that cuts off urine flow and get a feel for maintaining testicle position while clencing, then you can rock out as many kegal reps as you want.
In the meantime, to delay ejaculation, clench it off as you would normally... but since your testicles are drawing up also, reach down and pull them away from your body (or have her do it). The testicles need to be drawn up close to the body in order to ejaculate, so pulling them away and clenching works great to stop ejaculation.
Once you've isolated the muscles that elevate the testicles, and learn to keep them relaxed, you'll be amazed at how long your orgasms last. I've had some long, long, orgasms without "ejaculation" climax: 10-15 minutes easy... believe me - I've actually hyper-ventillated a couple times.
Keeping those muscles relaxed is much easier when you are on your back. That way, you have gravity to help keep the testicles pulled away from the body. I find that when I'm on top and moving, I tend to "mess up" and lose track of their position with all the swinging around going on, plus it's crazy stimulating to have them smacking against her... *SPLAT*
Indeed, bad form Kegals have a negative effect because they draw up the testicles TIGHTER to the body, thus making you ejaculate FASTER! If I am taking too long, I draw them up closer and thus speed things along.