This is a question we are asked on a regular basis in store. Firstly a couple of things to bear in mind.
1) Believe it or not your feet don't stop growing in your late teens. As we age our feet tend to splay out and take up more room in the shoe, so it is worth measuring your feet every once in a while to see what size you actually are.
2) The time of day you are buying your shoes can be important. If you are buying your running shoes first thing in the morning your feet have not swollen at all and will usually fit very different to the end of the day when you've been on your feet for the whole day. Ideally if you can try on shoes in the afternoon it gives you a better idea how the shoe may feel.
3) If you are only ever doing a 5k or 10k distance race you can usually allow the shoe to fit pretty tight because your foot won't expand that much during the course of the race. If however you are doing the marathon you may want to leave a little bit of extra room in the shoe to ensure that when your foot expands your not hitting the top of the shoe.
4) What brand of shoe are you running in. The brands can't even decide if there is a full size between mens UK and US size charts so they are by no means the same when it comes to fitting across all the brands. As a general rule New Balance drop down half a size for the UK sizing but most brands will drop down a full size. So just because your a UK9 in Asics doesn't mean you'll be a UK9 in New Balance.
5) Width- So many people get measured for a pair of shoes and only come away with one fitting the length but you need to have a look at the width. Many of the brands will do wider fitting shoes or just to get a brand that is wider fitting. For example many customers have a wide foot and could be in a very narrow fitting brand.
If you are having trouble getting something that fits you right, make sure the person measuring you up knows what they are doing. We can spend ages getting you into the right type of shoe but at the end of the day it's you wearing the shoe and it needs to be comfortable on your foot only you can know this.