Rigging smelt on the hook for rainbows? Maybe that's your problem right there, using the wrong bait?? I've never heard of anyone using smelt for rainbows....
But I'll answer your question, as best I can. Although I'm not sure what lure you are using, so I'll keep it generic. IMO, smelt is really soft and thus easily falls off the hook. I like to use salt to toughen them up. Using a brine like this also really helps:
http://www.salmonuniversity.com/ol_brining_herring.html. This will increase the strength of your bait.
If using a treble hook, I usually put the head portion over/through the treble and use a stinger to attach to the back of the minnow. The head is the strongest part of the smelt, usually hook though lips or eye holes. If using a jig, I usually cut them into pieces and hook them on sideways through the backbone. But if you have a small enough smelt and want to use it like a minnow rig, hook it like this:
Not sure if this helps, but let me know if you have other questions.