Offer the spread with a limit order at 50% of the bid/ask. Move it later if you're desperate.
I use optionsXpress which, while expensive, does a great job with its "walk limit" feature and usually gets me spreads or combos right around the middle of the bid/ask. You can customize the order to limit how far they take it, at which point it converts to a limit order. I've found that a lack of fill usually only happens on illiquid options, which I should have stayed the hell away from in the first place.