Wilkos, IKEA and charity shops. eBay and the rest of the internet hadn't been invented when I was doing up my house on a shoestring! Your local hospice shop probably has a furniture dep't.
Second hand markets. And car boot sales and auctions, if you have them nearby.
If you are at all Handy, get someone to buy you a sewing machine for Christmas/birthday, and soft furnishings will never be a problem again! New fabric isn't much cheaper than made-up curtains these days because of sweat shop labour but 2nd hand curtains and other fabric can be transformed very cheaply!
If all-white rooms don't appeal, paint one wall or even just a chimney breast with another colour to change the whole feel of the room very cheaply.
Carpet remnant/discount places for room-sized remnants, which are much cheaper than carpet off the roll.
Sales. Remember most places will have a Summer Sale any time now.
And - a boring but important tip! - budgeting is crucial. Work out how much you have to spend in total, then set yourself a budget to do up each room, plus a budget for 'special finds' if you want a bit of flexibility, and stick to it! (So for example, if you've got £100 to do up your living room, and you know you can't get carpet for less than £60, then you can't buy a £40 sofa AND paint the walls!)
Have fun! I really enjoyed those years (before kids) when doing up a house with no money was a creative challenge! :)