If you are showing people round yourself, let them into each room first, then follow them in. They are then entering an "empty" room which will make it feel bigger.
Agree with Pamina, declutter as much as possible - pack away all kids toys and paraphenalia, take down kids paintings etc. On your kitchen worktops, keep out only the essentials (kettle, toaster).
If you have a separate dining room and it's quite grand, you could consider setting it for a dinner party (might be a bit OTT if your room is small).
Look at your front door. If it needs painting, do it now. Front doors have a lot to answer for!
Make sure there are no wires trailing around. We haven't got enough sockets in our house and have extension cables everywhere - bad idea!
In your bathroom, clear out bath toys and big bottles etc - just leave out one or two expensive looking bath oils that match the decor of your bathroom.
When showing people round, talk about how friendly a neighbourhood it is/how good the schools are/how easy it is to park/how close the local shops are etc - anything you can sell to them as a bonus.
Plants. I have none because I kill them frequently! But make sure you have one in each of the main rooms, or possibly some fresh flowers.
Beware of cramming all the stuff you have put away into cupboards - many viewers will want to look inside to see how big they are. Put stuff in the loft/car or take it round to a friends!
Keep the rest of the family out of the way whilst a viewing takes place. The less people in the house, the bigger it will seem.