I like the Thomas and Betts zips that tbone linked to, though they do have a largish bump. I've found that it isn't an issue on my mouthpieces if it's on the side, but that's just me. The material is different and the stainless locking tab runs smoothly as you cinch it down - it doesn't click-click ratchet like other ties. If you over tighten, these ties tend to break cleanly at something approximating their rated strength. They don't stretch out like cheaper ties do, and perhaps fail later. Plus, I know what I'm getting.
I also have a couple of tools like Warren posted. They are nice because you can tighten with a "straight pull" without having to pull the tag end to the side, which often rotates the mouthpiece and leaves it cattywhampus. Just be careful. It is easy to overtighten the zip tie and either distort the mouthpiece or, worst case, crack the plastic underneath. You really don't need to have a mouthpiece zip super tight. Just a few clicks past snug is usually fine.
I also have a couple of tools like Warren posted. They are nice because you can tighten with a "straight pull" without having to pull the tag end to the side, which often rotates the mouthpiece and leaves it cattywhampus. Just be careful. It is easy to overtighten the zip tie and either distort the mouthpiece or, worst case, crack the plastic underneath. You really don't need to have a mouthpiece zip super tight. Just a few clicks past snug is usually fine.