Save up any money that you were going to spend and use it in the sales instead of in Primark. If you aren't fussy, the end of the sales can be really good - last week, I bought my children some tshirts from Gap which were £2.99 each, down from £10.
If you want a particular item, you could stalk it, until it's reduced. I saw a dress I wanted for £80, which got reduced to £65, then £35 and finally £25. This only works for stores that don't have the quick turnaround of top shop and h & m.
Department stores online can be an easy search under the sale tab as you can type in your size and you can see all sale items in your size.
TK maxx is good for a browse, but I normally don't have the patience for this. The homewares can be good. I saw some Joseph Joseph kitchen utensils for £5 and they sell in John Lewis for £10.
Ebay can also be good if you want to look for a particular brand. Many items are brand new with tags.
I take advantage of Sainsbury's 25% off clothing. I buy all the children's underwear from there when it's on offer and put away until needed. You can then spend the saving on something else.