I started by collecting items in a designated area. I systematically go through drawers, cabinets, closets, storage areas etc. I create 2 piles things I actually use, and things I don't. Be brutal, eliminate duplicates, broken, and anything unused in the last year. Do you need 2 spatulas? I do. Do you need 2 pastry cutters? I don't. Do you need 4 round cake pans? I don't. Do you need 4 9x13 pans? I do. Obviously your answers will be different than mine, that's OK as long as you say no and not just yes.
To your friends and family they may have items to add to your sale, they may volunteer to help set up, or sit with you, or even better yet they might want to buy your stuff!
Find a place to hold your sale. If you live in a remote location you will need to spend a lot more effort marketing than you would have to elsewhere. My house is in town with easy access from main roads.
When, is as important as where. Here we have an annual city wide sale that sees lots of people in town shopping and an organized list of sales to visit. Around here, it is also common for sales to end before most people finish work, so staying open late will allow for more people to shop.
You don't want to go through all the effort and have nobody come. Signs should be large with large letters and few words. They should be placed on busy roads and at constant intervals. Every turn needs to be marked and if the straight drive is long extra signs are need to encourage prospective buyers to keep coming. Post images on Craigslist and Facebook, run a classified ad. drop flyers at other nearby sales and agree to send people back and forth.
I couldn't believe how many people exclaimed in surprise how organized my sale was. Think about how your local big box store is arranged, you have a similar level of variety, so use their market research to your advantage. Make a clothing area, kitchen, decor, man stuff, sports, toys, books, clothes should be sorted by sized and well labeled with signs. Few people are willing to dig through stacks of clothes hoping to find the Holy Grail (stylish and correct size). Increase your customer base! Fold things neatly and refold throughout the sale. There is a reason clothing store workers are constantly folding and sorting by size. It works!
Unless you have extra helpers that will sit at a checkout table at all times (that isn't practical for me) use a 3 pocket apron instead of a cash box. 1 pocket for $1, a pocket for quarters/change, and a pocket for larger bills. Since the apron is always on, you can't be distracted while a partner swipes money from the cash box. It happens a lot, and is a valid concern. I keep my ones separate so that I don't accidentally give $10 in change when I meant to give $1. Know your bottom dollar. People will negotiate on prices be ready to counter or accept. Be sure to have $30-50 to start so you can make change. If making change scares you, keep a calculator handy. The easiest way to make change is to count it back to the customer. If the total is $13.25 and they hand you a $20 you will give them $.75, $1, and $5.
Since you have your money in your apron, there is no need to sit when customers arrive. Get on your feet, greet them, ask them if they are looking for something specific. I would hate for them to miss it, or they might be looking for something that I had intended to sell, but forgot to bring out. Engaging them in conversation will keep them looking for a longer period, making them more likely to make a purchase. It also has the added benefit of meeting new people in your community. Being on your feet will also help you spy items that didn't get prices, or need rearranging to appeal to the buyers.
Hopefully, after all your hard work, there isn't much left to clean up. box up all the items you want to donate, be sure to get a receipt for your taxes. Any remaining item should be listed for sale on a sight such as craigslist, and left in the garage until gone. Once something is removed from the house for sale, it shouldn't be brought back in, or it will go right back were you found it, only to be hauled out again in a couple of years when you do this again.