You should never use market orders before or after hours. Many of the same order execution rules don't apply and your liable to get a fill way out of the current market.
Even during market hours, it's now always a great idea to use market orders. I've always suggested to use price + or - a certain amount. For example, if your buying, buy at a limit price equal to .05 greater than the current best offer. Many trading programs will allow you to create a hot key or custom button that does just that.
Of if you're selling, sell at a limit price equal to .05 less than the current best bid.
If you use a route like ARCA, they will still attempt to match you at a better price than your limit price...but your protecting yourself down to a certain point.