What can I do to give myself the best chance of appearing well as a buyer?

It starts from the very beginning when you call up to arrange a viewing. Make sure to be polite, to attend the viewing and if you can't attend, try to give plenty of notice to cancel. When you do make an offer, it's important to not play too many games. If you're asked to provide information, provide it quickly. This will give confidence to the seller, and all of these actions will give an indication to both the seller and the agent of what kind of buyer they may be dealing with. If these interactions are all really positive, it builds an idea that you are someone who is pleasant, and easy to work with. However, if every interaction is hard work for the vendor, it could be the reason you lose the property.