Think of a theme for your evening. Yes, it's comedy. It should go further than that. If you decide on a definitive theme, it will be more catchy for potential audience members. If someone knows they are in for a night of dirty humor, it will attract an audience that genuinely appreciates that sort of thing. If it's a family comedy haven, you can get families out in huge numbers. Defining your evening is the first step.
Attend comedy shows all around town. Do this even if you can afford the big names in talent. The point of a showcase is to show off great comedy that is rather unknown or unestablished. it's a great gesture and decision to go with local comedians who are hilarious. This will be a great thing to sandwich in between bigger name stars. Of course, a no name comedian opposite Bob Saget is going to perplex audience members, but a clear introduction by the host should help set things up.
Contact bigger name celebrities after you have scouted local talent. If you can afford to go big, go big. People want to spend money on comedy that can be trusted to deliver a laugh. Talent that is unknown placed on a billboard will be followed by skepticism. If you can only afford one big name, try to get it.
Visit all local venues. You want to choose the best possible location for your show. This will also be decided by Step 1. If you are doing a family show, doing it nearby a school in a safe district is a must. If it is on the darker side, this can be placed appropriately as well. Location is key in real estate, and it definitely does play a part in attracting audience members who like to hang in the local neighborhood.
Process all the paperwork. Send (or have your lawyer send) all the necessary paperwork and contracts to the talent, along with any money if relevant. Some new comedians will do it for experience, especially if there are stars involved. However, you don't want to make a fortune off the back of the struggling comedians. Pay everybody fairly. In doing so, you'll be protected in the long run, and you can also sleep at night. It's a win-win situation.
Send out a press release. The celebrity act (if acquired) should be listed in the return mailing label (along with your name). You should have the celebrity's name at the top of the paper. List all the comedians along with a brief biography and review of their comedic style. Include some really interesting information about your showcase, including funny stories that happened along the way. Include every bit of newsworthy information that you came up with in Step 4.
Promote your showcase. Call radio stations, local television news stations and all the newspapers near your town. Try to get interviewed, covered and asked about your project. The more people know about it, the more people will be in attendance at show time.