If you need SEO services there are a few things that you need to consider before you buy. The first major issue with many of these services is that they can be very expensive. You should definitely shop around when looking for services to buy. Many companies sell the same type of service and you can often get better deals looking for a similar or related service instead. The second thing you want to look out for when looking at SEO services is how legal they are. Some services are actually just spam and basically they spam the URL of your website all over the Internet. That description sounds a little dramatic, but depending on the service it can be closer to the truth than you think. In order to avoid these types of services all you have to do is ask the company how their service works. If it sounds a little shady to you, then it probably is. The third thing you want to look for when buying search engine optimization services is the longevity of the service.
Search engine rankings change all the time (literally on a daily basis). So the service that you bought last week that was supposed to be effective, might not be working that well this week or in a month or two. It's not really the service providers fault. Most of the time the way search engines decide to rank websites changes a little bit and the service that was being offered simply becomes less effective. There are two main ways to avoid this type of situation from happening. The first is to make sure you contract your SEO work out on an ongoing basis. This is so whenever the search engine algorithms (how they rank websites) change, you can be sure your SEO strategy will change with it without having to go shopping again.
The second major way to avoid your website getting hit hard from an algorithm update is to purchase services that have had proven results for long periods of time. If a service you are looking at has a long list of clients that it's worked for in the past, than chances are it is probably a good buy and will work in the future as well. This isn't as safe as making sure any SEO work that is done on your website is done on contract, but it is cheaper.