1. The first thing you have to do is to check out the selection of servers they are offering. VPN servers are basically secured connections which are meant to shield you from any type of hacks. This is done because all of your information is re-routed through a VPN server where its integrity and security is ensured before it is transmitted onto your computer or vice-versa. You are basically insulating yourself when you send all your communications through a VPN service provider.
2. The next big determining factor should be the price. But don't only compare the price you are paying to various service providers, you also need to compare the bandwidth. You would not want to reach your bandwidth limit on a regular basis now would you? There are even some service providers that offer unlimited bandwidth. Perhaps you could find one of these instead?
3. Checking the compatibility of a VPN service is very important as well. If you want to use your VPN network on your iPad but the software doesn't support iOS, it really won't be of any use to you. Some people just go with the cheapest possible plan only to later on realize that it is not compatible with anything apart from your old desktops. What a waste of money right?
4. And lastly, you need to check the additional features being offered by your VPN service provider as well. Will they allow P2P usage? You would not your VPN service to protect you from malicious attacks by blocking the websites which you want to visit now would you? It is important for to not only speak with the service provider regarding this but to also get it in writing so that there is absolutely no doubt as to what you're getting into.