The biggest factor that you should check before going in for hair enhancement procedures such as putting on extensions is the quality of your hair. If you have very weak or sensitive hair which cannot take the push and pull weaving might involve, don't go for extensions through the weaving method. Glue is not the most preferred method but at least you do not damage your hair.
Not all stylists are good at putting up extensions and only the ones who have received hands on training to do the job are good at it. So get a stylist who is trained for hair jobs even if that means paying more, because saving up here will not give you what you want. There are also frequent maintenance visits to the salon to keep your extensions looking as good as new. Sometimes they cost as much as the first visit or only a little less. Make sure you make your budget keeping all these costs in mind. There is no point worrying later as then you will end up compromising on your hair style and your look.
Last bit is the time. It takes minimum four to eight hours to put up the extensions and some might even take longer. In today's busy world sometimes it is difficult to take out so much time at once. Check on your monthly calendar before you go for hair extensions and be ready to shed some time to get the perfect look. A hurried job is never the best one.
Next is the kind of extensions you should buy. Always invest in quality natural hair extensions because a) they have more shine due to the preserved cuticles and b) they are absolutely tangle free, also c) they can be styled more freely than synthetic hair which might not fare well with ironing or curling. Buy them from good stores or you can even buy them online. There are plenty of stores on the Internet and all of them provide quality extensions. You can read about them, check on them on forums and buy when completely satisfied.