I'm pretty late to the party, so I don't know if you and your hubby have taken your adventures to the realm of Soap Stick Land already, but here are some tips.
Like others have said, be careful with soap that could cause irritations or bleeding. You do not need highly aggressive soap for soap sticks to cause discomfort, especially if you have never used them before. Ivory and Dove are middle of the road. Soap sticks made of those brands will burn, but not enough to harm your rectal area.
Use a small, sharp knife to cut a bar into two or three pieces lengthwise. Round off the sharp edges with the knife, then rub the sticks down with a wet washcloth. Voilà!
Right before use, let your husband hold the tip (or half) of the soap stick in a cup of warm water. Warm, soft soap burns that much more than dry soap. Let him SLOWLY insert it into your anus. Being slow is much more uncomfortable when it comes to soap than being rough. Insert the stick halfway, leave it sticking out of the anus for a couple of seconds, withdraw, re-wet the tip, insert again. Moving a soap stick in and out of the anus will build up the urge to go a lot more.
Once there is a nice layer of soap, your hubby can push it all the way in, deep into the rectum where it will really kick in. Of course, after all this, while you are squirming, he can then use another stick... and another. 😉
Good luck and safe playing!