I have been into art and antiques for over 30 years now, and have talked to some of the best experts in their fields, and have found the two basics that they can all agree on are.

1.) Buy what you like, what makes you smile when you look at it, and brings you joy and satisfaction. Not for it's potential investment value, which can change quite quickly. Unless you are buying strictly for monetary value, in which case you should know the market for the type of item your buying, and plan on selling it quickly. Because if you purchase an item  mainly for it's potential investment value, and the demand for that item suddenly drops, you have to live with it.  

2.) Buy the highest quality, in the specific catagory, that you can afford. For instance, if your passion is porcelain, and say a particular house or company, save up and buy the best piece, rather than several pieces of  lesser quality. In particular avoid damaged items when looking for glass or porcelain. Even if such pieces are professionally repaired by a qualified restorer, that restoration will be detectable, and therefor never have full value. With antique furniture, remember a piece is only as old as it's most recent restoration/replacement, ie. replaced hardware, such as drawer pulls.

By keeping these two basics in mind, you will never be disapointed with your purchase.