1. Where you shop
Vintage shops: These are definitely the easiest places to pick up vintage pieces. Places like East London and Brighton are swimming in vintage shops. I strongly recommend Beyond Retro and Judy's Vintage Fair for diverse stock and fair prices. The problem with vintage shops (or vintage markets) is that they're often overpriced. Also, be aware that just because it's a vintage shop, it doesn't necessarily mean they'll be selling genuine vintage, but more on that later.
Charity shops: These are much, much cheaper. You can pick up some real gems in charity shops since they're less frequented by vintage-hunters. The downside is that it's pretty rare to find genuine vintage and you might need to spend a long time rummaging through Primark tat before you find something.
Online: Without doubt, the most convenient way to shop. Etsy is fantastic. It has thousands and thousands of vintage pieces from all over the world. Just type in your desired era and see how many results you get. You're bound to find a bargain too! Be very careful with sizing as you don't want that perfect piece to be a bad fit when it arrives! Also make sure you check the photos and description carefully for flaws and beware that if you order from abroad, you could be landed with a customs fee to pay.
2. How you shop

Ok, you know where to go now. You're at the doors of the vintage shop, between the rails of the charity shop or sitting in front of your laptop in a onesie. Now what? You can either quickly flick through and go by immediate response, or study each piece carefully to consider it. I prefer the latter but it's personal choice.
My advice, however, is to broaden your search. If you're in a shop, try things on, even if you don't think it will fit or if you're not sure of the shape, print, whatever. It might surprise you once you try it on! Don't limit yourself- Look in menswear (womenswear, if you're already a man!) and childrenswear. You might find that perfect piece!
3. Know your stuff

Right, you've found your perfect piece. How do you know that you're paying for genuine vintage? Well, there is a lot of information on this, far too much to outline here, but here are the main questions to ask yourself.
Is it homemade? Before the 70s, when ready-made fashion really came into prominence, a lot of people made their own clothes at home. If there's no label inside, if the fabric has been cut with pinking shears and there's no overlocking, it's likely to be pre-70s.
Is there a label? As I said, from the 70s onwards, clothes were more regularly bought from shops and therefore labels appear more frequently. Of course, clothes did also feature labels before this too. Check the lettering and familiarise yourself with stylistic features of each decade. Union labels are great because they provide a very precise way to date. Google the brand name, company name or any other specific information that may appear on the labels.
Is the zip metal or plastic? This is a biggie. Plastic zips were very rarely used before the 70s so big chunky metal ones are likely to date from before that. As with the other tips, though, this isn't a guaranteed way to date clothes although it is a good guideline.
4. Check it over

You should already have checked it over in order to date it. Whilst doing so, look for flaws. Are there any spots of discolouration or little holes? Look for seams that may be liable to split. As disgusting as this is, check out the armpit area and make sure it's not yellowed or, even worse, smelly! Of course, vintage pieces are likely to have imperfections. A hem that's falling down is easily sorted out and small stains at the waist, for example, might be overlooked or covered with a sash.
5. Get a good price

Right, you have your piece. You've dated it and checked it inside and out. Now you're ready to buy. How do you make sure you pay the right price?
After buying a few pieces, you'll pick up a guideline price. Generally, I pay no more than £20 for skirts and knitwear, £25 for dresses and £40 for jackets as long as they're standard day clothes with no special features. Of course, that price goes up if it's something really spectacular.
With vintage markets and smaller vintage shops, you might be able to haggle with the owner and get a better price, especially if the piece is damaged. Be aware that larger shops with multiple employees may not be able to do this and it's considered bad form to haggle in a charity shop.
Ultimately, it boils down to what you're happy paying. If you love it, and it's affordable for you, go for it!
So there you have it, my top tips for vintage shopping. I'd love to hear your adventures of vintage shopping, and let me know if I've helped you on your vintage shopping explorations!