I have openly stated that as teen I do not have a huge disposable income so I do spend a lot of time researching not only what I intend on buying but any beauty related gifts too. As a student money can be tight and I want every product I earn its place within my collection. I'm sure even those with a huge salary will share the same sentiment - waste not, want not?
Normally my first port of call is to ask my sister if she owns the product in question (one of the many perks of having a beauty blogger as a relative), and if I could sample a little. If not I'll quickly Google for reviews and if it is a make-up product swatches. Sure what one person loves, another will loathe but generally reading a handful gives you a fairly good feel for the product.
If I like the sound of what I am reading I then either pop in-store or on counter, yes high end skin care comes with a hefty price tag but in my experience most brands are more than willing to give you a sample to try, before you buy. Obviously high street brands don't have the means to do that so beauty reviews really do come in handy, even then it is still a gamble with skin care. Why can't beauty shopping always be as easy as purchasing a fragrance? One quick sniff and you know there and then if you are a fan or not! Make-up shopping is slightly easier - always make use of the testers on your arms and hands (never your face in the instance of lip and powder products), test foundation on your neck and step outside into natural light before purchasing (damn those deceiving fluorescent strip lights).
I know it is easier said than done but do feel free to say no to pushy sales assistants, I know that they are only doing their job and have sales targets to reach in some instances but it is you who has to hand over the cash. I normally find that explaining I am a poor student does the job as most can related but don't be afraid to walk away from the counter. In the same vain feel free to not sign up to store credit cards, no not those that reward you with points each time you shop but those that give you a credit balance to spend in-store. What may seem like a great idea is normally steeped in interest and will take a long time to pay off, you could still be paying for that pricey bottle of foundation in two or so years time. Far better to save up and wait (I always feel so accomplished when I save up for something) than be tied to one of those store cards if you ask me, of course to each their own as we all have different needs and reasons to do so! If you are looking for a solution to pay off the debit so to speak you could transfer the balance to a 0% interest balance transfer credit card to help pay off the debt quicker and easier.
If you do purchase something that isn't for your needs, then why not have a monthly get together with friends and have a swap party? Obviously this will work better with some products than other but if you hygienically cleanse the products I see no harm, heck you may even get your hands on that one product you've coveted?