You will want to ask people you know who they have to get an idea how service is. Next you need to evaluate where all you travel to get an idea where that carrier serves. Look at all the plans, and phones. Also read the terms and conditions of each wireless service. Regardless which carrier you plan to go to, it's best to go through the corporate wireless store rather than the authorized agents as the authorized agents don't always have the best knowledge about the service they sell, some authorized agents may not have a large selection of phones, and in the case of a Schererville, IN authorized agent of Verizon Wireless service, this store sells a number of discontinued phones as new, sells car chargers that aren't approved by the manufacturer, and the sales person had a major attitude. You'll usually pay more at the corporate store, but at least you're dealing with someone who has the best knowledge of the wireless service.
I'll give you my criteria of what I looked for in a wireless carrier as an example:
I looked at the number of minutes I would use a month during peak period. I also looked at the coverage maps of where I travel, and for me I travel through lower Michigan, northern and sometimes central Indiana, Chicago and nearby suburbs, and in Wisconsin around Milwaukee and occasionally Madison. Based on where I travel, only Sprint and Verizon Wireless met my needs, but I chose Verizon Wireless for the Americas Choice plan after noticing Verizon Wireless's coverage map was larger than Sprint, for both Verizon's network and select roaming areas that they call extended coverage. Verizon Wireless has done great for me for the last 2 1/2 years and plan to keep Verizon Wireless as long as I live in their service area.
Before I forget, you'll want to look at what kind of phone you want. If you can't afford a top of the line phone, a budget phone from Nokia or Kyocera will give you the performance call quality and signal strength wise. Both phones handle most weak signals rather well. I find that to be the case with my Samsung SCH-a650, which I upgraded from a Motorola 120c, which dropped too many calls and couldn't handle a call with less than 3 bars.
I am currently an AT&T Wireless customer and my contract will be expiring soon. I am evaluating cell phone providers and I am trying to determine the order of importance for making the selection. I am looking for feedback about signal strength and customer support. Utlimately, what are the deciding factors for picking the right cell phone provider.