We've successfully sold our last two homes very quickly; in one week for more than asking price on home #1 and in one week for just less than asking price for home #2.
I agree with the other posters. The biggest thing you can do is clean and declutter. Go through each room and get rid of anything that you would get rid of when you move, anyway: old clothes, furniture, papers, toys, junk in the yard, etc... We spent weeks just decluttering each house. Then, clean the dickens out of the house, top to bottom. Then, decide if there's any maintenance you need to do: repair broken items, touch up paint, etc. Don't worry about remodeling dated bathrooms or anything major.
Once, you've gotten rid of the clutter, cleaned, and fixed maintenance issues, do what you can to make the house feel generally "homey". In our first house, there weren't any built-in light fixtures and the house felt dark. We went out and bought several different inexpensive lamps. We checked each light in the house and made sure that we were using the highest allowable watt bulbs. We hung a large mirror in the livingroom to add the impression of even more room and light. Then, before a showing, I would go through the house and turn on every light (this was during the winter). Boy, that house felt warm and cozy with every light glaring. We also got those "plug-in" air deoderizers that were vanilla scented and used them in different rooms throughout the house -- not enough to be annoying, but added the smell of baking without the mess.
In our second house, we had an unattractive multicolored carpet throughout much of the house. The carpet wasn't half bad in most of the house, but didn't look too great in the livingroom which had great views and lots of light -- the carpet just looked too informal. When we first started thinking about selling the house, we had our realtor take a look at it and she pretty much insisted that we needed to replace the carpet, which was only a few years old. Instead of spending thousands on new carpet, we bought a solid, cream-colored area rug for a couple hundred dollars. When we finally went to put the house on the market a few months later, the realtor didn't even remember having had a bad opinion about the carpet in the livingroom and thought it looked great. Again, in the second house, we made sure that paint was touched up, used electric air fresheners, and we got rid of the nasty-smelling diaper bucket.
I don't think you need to hire someone to "stage" your house. Get a few home magazines like Better Homes and Gardens and flip through them to get ideas on how they've staged those homes -- no clutter on surfaces, no toaster on the kitchen counter, not too too many family pictures, etc...
So, a few things that may be worth the investment: a few coats of paint, if and where necessary; lamps and/or higher-watt lightbulbs; air freshener; repairing minor maintenance issues; some inexpensive plants in the front yard; a nice plant or flowers for your diningroom table.
When you're selling your house, it should be the cleanest it has been since you bought it. You are selling the buyers the delusion that if they lived in this house, they too could keep it just as clean.
If you're looking to sell rather quickly and painlessly, get an experienced real estate agent and let him/her know that you are interested in selling the home quickly. IMO, many people are interested in making a few thousand dollars more instead of SELLING the house. An experienced agent can run the comps and tell you exactly what your house is worth -- price it accordingly.
Something that we did when we sold our last house that worked VERY well and we will do again with any future homes we have to sell and I HIGHLY recommend: When we had the house completely ready for the market and our agent had everything lined up to put the house on the MLS, we left town for the first week of the home being on the market. We had given our agent a couple week's notice that this is what we planned -- we wanted to be out of the house for the first week the house was on the market and wanted her to get as many people to look at the house during that time as possible.
We had a toddler and an infant, at the time. Instead of worrying about keeping the house clean and worrying if someone would want to see the house during naptime or dinner, we went on vacation. Yes, our house sold while we were away on vacation. Our agent had all of our contact information and we checked in with her for status updates every couple of days. We had an acceptable offer before we ever came home from our trip.
Good Luck selling your home!