Tips

Take a before pic!
Pick off "low hanging fruit" first- throw away or recycle trash before starting anything else
Set a timer (between 15 mins and two hours), then take a break when time is up
Any item you come across where the action will take 3 minutes or less, do it immediately, otherwise create a to-do list
Keep a basket near-by for items to go to other rooms in the house, don't take them one by one as you come across them
Take photos and measurements of areas where you think you might need to purchase a furniture piece or organizing product
Show off your progress to a friend
Acknowledge your wins and achievements, this can be hard work