Have a theme in mind
Once you decide on a theme, you will end up looking for decor ideas and items restricted to the theme. Hence, it will spare you of the temptation of picking up items just because you fancy it. If it doesn't go with your theme, don't buy it and this should be your rule when you shop.
Rearrange the furniture
Decorating doesn't mean you have to buy new stuff. Just shifting your furniture around the room will definitely give it a new look. If you have two similar looking pieces of furniture together, put them in opposite corners. Break up three-piece sofa sets with coffee tables or potted plants.
If you've always had bare walls in the room, you can give it an entire new feel by adding a couple of decor items on the wall. You don't have to go in for those expensive paintings; just frame up memorable photographs and randomly hang them on the walls. If you have kids, get them to do some art work and frame it up. Make sure there's a lot of colour, it will brighten up the room
The 'green' factor
Potted plants add a nice burst of energy in a room. Moreover, plants are relatively inexpensive and all they need is a bit of care and attention. Get a variety of plants - some big ones, some creepers, some flowering plants and an assortment of cactus pots, these are perfect to place on a table top or a window frame.
Used furniture is a treasure trove of inexpensive items that you can pick up for your home. All you need to do is re-varnish or polish the used furniture and it's as good as new. If you're lucky you could even pick up a couple of antique pieces for a steal and place them in strategic corners of the house.