If you're awake when you wet, roll to your back as the back usually has twice the padding as the front. If you're on your back and you leak out behind your legs, then the diaper just isn't very good. But you can pull your knees up a little to help with that. Let the wetness soak in for a minute or two before moving around too much or you'll just squeeze it out before it has a chance to be absorbed or turned into gel (if your diaper has gel granules).
I'm a bed wetter and sleep on my tummy a lot and a mom gave me a great tip yesterday. When you're changing you can use one of the wipeys or a dry gauze pad is better and place it over your penis, you can wrap it up basically. Then put the diaper on. Then when you pee, it diffuses the stream so it's not so powerful and sends it in different directions a little slower giving the diaper a better chance to absorb it all quickly instead of just concentrating on one little spot.
I tried it last night, and I'm not awake when I pee, but it seems like it worked because I wasn't soaked to the top of the front pad like I usually am, and I was just as wet in the rest of the diaper as I usually am so I know peed the usual amount.
Also the extra layers are a good idea too. When I'm nervous about the bed I'm sleeping in, like hotels and such, I wear one of those terry cloth pull on diapers and plastic pants over that. That way even in a pretty good leak from the disposable diaper, the cloth diaper over it will soak up quite a bit and then the plastic pants obviously make it waterproof so the bed stays dry.
Instead of plastic sheets, you can get a cloth type mattress protector that's still waterproof. They're silent under the sheets and you can't even tell its there. Even for none bed wetter they're a good idea because they protect your mattress from body oil, dead skin, sweat, etc. Your wife probably won't mind that because it'll let your mattress last many more years than normal, so you save $$$.