You're looking to sell items which have a consistent fluctuation in the event that it makes sense. Things that increase and decrease, but RS gold predictably. To know when things are likely to be able to rise or fall out of this time frame, keep an eye out for large updates, such as days with double xp or new quests. Make sure to sell items that most people will use to enhance their skills. Do it in large quantities.
When buying on the exchange, I tend to go between 5% and 10% less than the median price. It is then possible to sell it as soon as the price goes up 5%-10% above, or you could come up with an individual "wholesale" price, which will generally draw more attention from selling outside the g.e.
If something doesn't sell or buy right away, don't be anxious. If it's not a purchase this is because you're paying below the average price, and the only people who don't care and are desperate for cash sell under medium. The item is likely to become available at the end of the day.
If it's not selling then it's likely to be an item that's not in demand or you've chosen an item that isn't worth the price, as well as the value you've set is too high. I wouldn't recommend taking the price down until a week or two after you have put cheap OSRS GP your offer out. Rememberthat merchandising doesn't happen overnight. It takes alot of patience.