Many of us buy our clothes on line now and so it's a good rule to buy two sizes if you are unsure of the fit of what you are buying. One can be returned later and it will give you a good idea of the brand's sizing - whether they are on the larger or smaller side.
Here are some pointers for ensuring a good fit with your clothes:
Trousers and Jeans: Make sure the length of your trousers and jeans is correct. If you are going to wear heels with them, the trousers need to partially cover the foot; if you are going to wear flat shoes they can be a little shorter. They also need to fit at the waist or hips with no bagging. You can, however, always wear a belt to bring them in at the waist or hips, but allow for this in the length. They shouldn't be too tight either to produce the 'camel toe' effect or a 'muffin top' if the jeans or trousers are low rise.
Skirts: Ideally the waistband should be loose enough to fit two fingers between your body and the waistline. Too loose and the skirt will move around; too tight and the waistband will bend over and there may be spillage over the top. Best to get a larger size and cut the label out.
Jackets and Coats: Make sure the fit is good at the shoulders. The shoulder line of the jacket shouldn't extend further than your shoulder, nor should it be narrower, which will also very likely mean that the sleeves will be too short. The sleeve length should end at the break of your wrist and make sure you try on a jacket or coat with the thickest item of clothing that you will be wearing with it, because the sleeves will ride up with a thick jumper for example. You should be able to do the buttons up on a coat or jacket, the exception being a denim jacket.
Shirts and Blouses: The obvious fitting problem is - does it gape between the buttons at the front? If so, then size up but bear in mind the other points for fit at the shoulders and cuffs. If you have a large bust, it may be better to go for a blouse or shirt that pulls on over your head with a collar and open neckline, giving a shirt effect such as this one from Kettlewell Colours
or this blouse from Boden:
Dresses: In my opinion, you should only wear a body con dress if you are a size 8/10. For the rest of us, a slightly looser fit looks better if the dress is straight cut or a shift dress. Please do bear in mind your body shape when choosing dresses. If you have a small waist and larger hips, your dress should be shaped to follow the curves of your body. If you have a straight body with little or no waist, then you can wear a straight cut dress. The armholes of your dress shouldn't be cut too low and the dress shouldn't pull across the bust.
Shorts: Follow the advice for trouser and jeans above, ensuring that the length of the shorts is flattering for you. Too short is never a good look unless you are young and/or really slim!
Do have your clothes tailored to fit you if they are not quite right, especially in the length. In my experience, you are more likely to wear the item of clothing if it fits you well.