I use the term rules loosely - truly. As far as I know there is no definitive book on how to decorate (if there is would somebody send me a copy?!). These are just tips and techniques that have worked well for me in the past and I use when helping out friends.
Style: save a lot of headaches, define your style in writing and be specific. My style is Mid Century Modern (clean lines with subtle curves, simplicity, organic and geometric forms, texture, wood) with a color scheme that is dark turquoise, light aqua blue, olive green, brown, and cream with black accents; a focus on folk or hand made art and accessories; live plants and comfortable functionality. That's my entire house. (OK, my condo is small, but this works with big houses also). When I'm out shopping and see an adorable purple silk fleur de lis pillow that is 95% off and now only $1.25 !! I know to walk past it because it won't go anywhere in my house. Confused what your style is? Watch for an article about learning to define your style coming up.
Link your rooms: repeat colors, fabrics or themes in various combination so all your rooms flow from one to another. If you do this, you can "shop your house" and move accessories around from various rooms and it will all work. You can also pick up that fantastic pillow on sale, even if you don't have a specific place for it in mind, because you know it will go in multiple rooms.
Inspiration piece: pick a fabric, rug, plate or something beautiful to focus your decorating decisions. (OK, this means before you start decorating. It goes along with defining your style). It should reflect your color scheme, style and mood of the room. My issues with my decorating crossroads post were because I didn't pick an inspiration piece and stick with it.
Repetition: If you have an orange accent in a room, repeat the orange color at least 3 times so it doesn't appear as a misstep. Use an imaginary triangle to place the items in the room for interest. You don't usually want 3 orange things clumped on one shelf and no other orange in the room.
Accessorizing: use odd numbers (3, 5, 7) and place items high, medium and low. Again use the imaginary triangle, but this time a wonky one.
Function over fashion: make sure there are end tables for drinks, lights to read by, and clear pathways. Pillows are great and all, but if a guest has to move pillows to the floor in order to sit down, you have too many pillows (gasp! - bet you didn't know there was such a thing).
Carry samples: keep your inspiration piece or a picture of it, paint samples, fabric swatches and room (or couch or wall) dimensions with you all the time, even if you aren't specifically shopping for an item. Some of the best buys are the unexpected ones! I have mine in a cute photo album/clutch. I try to have it in my purse always. (OK, this isn't a lie. I try to have it with me all the time. I just don't succeed. And when I don't have it I can't tell if an aqua has too much gray in it to go in my home, and then I have to leave the item, and then I think obsessively about said item, until I eventually go back to the store with my swatches, and said item is now gone, so I think obsessively about the one that got away - true story about a bath rug.) It even has some inspiration photos to help remind me of the look I'm trying to achieve.
Look at your room with an editing eye (as Tim Gunn would say). If you have an item that doesn't fit in with your defined style or is completely out of sync with your inspiration piece think about replacing it. Unless you are going for an eclectic look this is good decorating advice but might not be practical life advice. I realize this doesn't always work in the real world when you have family heirlooms, children's accessories and toys or even hand me downs that you can't afford to replace just yet. Looking at your room with an editing eye will help you evaluate your existing room and develop a plan on where you want to go next.
What are the basic decorating "rules" that you find helpful?