NB: when buying extensions there are many types of hair extensions on the market. There is huge variation in Quality. You need to know what type of hair is it ie. Asian, European etc, and what that means exactly. Where did it come from. What Chemical Process has it been through. If someone is claiming that it has not been through one DONT BUY. All hair must be processed if only to make it through customs but also to be coloured or striped and coloured. This is not a bad thing and may actually improve the hairs condition. You do not want hair that is over processed however this is when the cuticle is stripped away and then the hair is coated. You will know this hair by the off colour it turns when u apply hair colour to it..usually a greenish col. Also the length of the hair and how much will you need to blend the length into your own.. if there is not enough hair you will see a line through it where your own stops.
MOST IMPORTANTLY; How is the hair going to be attached to your own. Glue is not recommend the hair usually sheds leaving behind the glue in the hair. it is difficult to remove and its offen recommend to use asetone which actually eats at the cuticle and causes damage to the hair. Metal clips hav pro's and cons... The pro's are that they have little shedding and can be moved up the hair..so they are reuseable. the main cons are that metal can cause breakage and also reacts to colour. i have seen cases where months after having these the person goes lighter and ends up with blue patches where the metal clips once were. The best bet it u can find the REAL thing is a KERATIN BOND.  Keratin is a protein that your hair and nails are made from and some companys hav mimiced this in there bonds. The pro is that keratin will expand and contract like the hair itself does with hot cold dry wet etc. So there is little to no stress on the hair. Keratin is usually removed with alchol based solutions making it no more harmful then hairspray. the cons are that the good quaility ones are more expensive due to the technology n they are usually prebonded making them not reuseable but in most cases they will out live all the others by several months and contain the best hair with NO SHEDDING.
For those who r serious- DO YOUR HOMEWORK! Greatlengths.com have really great info on types of hair, quality, home maintainace ext. And you are alway best to have them applied buy a qualified hairdresser.