I am a mother of two (25 months and 5 months) and so have had a fair bit of experience buying clothes for them (perhaps too much...)
Along with all things child related, there are things to be learnt following mistakes. Here are some things I have learnt the
hard way, or been lucky to stumble across before wasting too much cash.
Designer Clothes: Babies don't need designer going home outfits from hospital. I heard of one girl who bought something ridiculously expensive for the photos, but found the outfit was highly impractical. Babies look cute and comfy in babygros. Leave the poor baby be! Expensive designer babygros tend to have really stupid popper systems, like ones where the seams turn inside to hide the poppers at the legs. Surely these aren't comfortable. Avoid any babygro that his poppers up the back (poor child), anything that only has poppers at the legs and not up the front (babies usually hate things going over their faces) and any that look difficult to fit. You will end up having a lovely babygro to look at that your child will never wear. The expensive ones I have bought / been given often seem to come up small or shrink on washing, so you hardly get any wear from them either.
Babygros: Babies still only need vests and babygros until they are a good few weeks / months old. They don't need to look like little adults for a while yet.
Don't shop too early for clothes. Children have a habit of not conforming to our expectations, so that gorgeous jumper that would be just right for winter may either be too small by autumn, or will fit perfectly by the middle of next summer. As an example, my daughter, Leala, is two and now wearing some of the clothes she was given as a present when she was born. She has been in 9-12 month clothes for nearly a year (handily hitting all of the seasons!). Nyle, her baby brother, is 5 1/2 months old and wearing 9-12 month clothes... If you do get tempted in the sales, buy items that could be used all year round - t-shirts, medium weight trousers, cardigans etc.
Layering: Linked to the point above, when you shop, go for lighter weight layers. I will dress my children in a vest, t-shirt, light jumper or fleece and then a cardigan over the top, and perhaps a jacket at in the early spring. Depending on the temperature outside and inside, I can then easily adjust the layers without too much hassle.
Pre-loved (OK, second hand). I only buy clothes from the higher end retailers from charity shops, e-Bay or car boot sales. The quality means that they last well and I am always chuffed that I have got a good item for a fraction of the price. I also don't get too stressed when, on the first outing, tomato sauce is spilt all down the front of it. I have had some great bargains, and I intend to sell clothes on when my children have finished with them. Second hand clothes go so cheaply that I won't feel disappointed if a vest I paid 25p for goes for 10p, whereas I might do had I spent £2 or £3. If you can, keep an eye out for 'Nearly New@ sales. In the UK, the National Childbirth Trust (NCT) organise sales quite regularly and the clothes are generally good quality and well priced.
If you are fortunate enough to have friends who have children, why not take them up on any offers of clothes. These might be on loan or even a gift. Just make sure that you know beforehand as it saves any embarrassment later. Agree to mark the labels or make a list of any clothes you are borrowing, or perhaps just note down how many vests / t-shirts etc you have an agree to return the same number. I was fortunate in that I had a friend who had each of her children nine months before I had each of mine. She lent me things as her kids grew out of them, and I was able to do the same. I definitely got the better end of the deal! I had other friends and relations who gave me clothes their children had outgrown, and I generally had an agreement that any not worn or outgrown would be passed on to someone else with young children or would go to a charity shop. I could then pick out the ones I liked and choose not to dress my children in those outfits that really didn't go with my taste.
Posh Stuff: If you have to buy lovely, new, expensive stuff, try to protect it (haha!) and then sell it off at the end. Look for discount codes on the Internet, join loyalty schemes and cashback sites to make the most of your money. Maybe you are fortunate enough not to have children that like to wear their food as much as eat it!
Quality not quantity: If you are more restrained with your spending, or you have the cash, why not go for quality over quantity. You can buy a few outfits knowing that they will last well and wash well. If you don't have financial issues and you aren't the sort of person to worry about the odd stain, go for it. Many declare that cheap clothing is a false economy. However, whilst I agree cheaper clothes don't always look quite so good, I buy a few more and so they get less wear. I have not yet had to throw anything away after wear from just one child.
Sizes: Learn from friends what sizes are like from different shops. This may sound strange, but sizes vary wildly not only across different shops but even across ranges in shops. I have found that Leala is in 9-12 month clothes from one shop (Boots), yet more like 6-9 months from another (Asda). If you have a good idea of the sizes, you are less likely to be trekking back and forward to shops to change things that don't fit or that the child grows out of quickly.
Keep a list: There are just so many lovely clothes out there that even buying from cheaper shops and supermarkets, I get tempted to buy. I am now going to keep a list of things that my son and daughter NEED, and carry it around with me. If it's not on the list, I'm not buying it! OK, this may fall by the wayside occasionally ("my name is Helen, I am a child clothing shopaholic"), but I'm going to try.