1. Don't get rid of something just because it isn't in fashion at the time.
I have done this many times, you're ruthlessly going through your drawers, find that black lace top you never wear, throw it in the charity bag. Then a couple of seasons later you want it back. (I still miss that top). Fashion often repeat itself so to save yourself from buying it again, just don't throw it away, just in case. This is also the case with my tartan skirt, I bought it last Winter when tartan was in and this year it's back! I'm happy about this because I love my tartan skirt.
Simple DIY customisation on clothes can make them look brand new and for hardly any money. For example when the whole 'putting spikes/studs on everything' came in, a lot of people just added their own spikes or studs to old clothes because they're only about £2-3 in shops or eBay. I tried to do a bit of the DIY thing too, when it was the yin yang phase I drew them on my shorts, but I'm not sure how great that looked. I also added beads to the collar of a dress when beaded collars came in - a more successful DIY. All I did was sow little beads to the corners of my collar and it made the dress look a lot less boring. So you don't need to change your whole wardrobe each season, just change the clothes you already have.
3. Buy one piece of clothing that's in trend to go with your old clothes
This summer I bought a skirt that fitted in with the whole monochrome idea but it was the only monochrome thing I owned. However, it goes with so much, I can wear it with vest tops, bralets, jumpers etc, so I can do monochrome in lots of different outfits with one skirt. This also happened with the pastel phase. I had a couple of pastel pieces like a cardigan and a tshirt then paired them with shorts or jeans so it was still following with the pastel trend.
4. Take a look in a charity shop
Charity shops can be full of all different and exciting things and you never know unless you look. Sticking with the same idea that fashion repeats itself, if someone donated some clothes that were in a few or maybe even ten years ago, then eventually they will be in fashion again, so have a look and you may find something in trend and at a reasonable price.