If you're buying an apartment as an investment and intend to rent it out or only reside there part time, you will need to focus your search on condos, as very few co-ops welcome non-resident buyers and almost all severely restrict the ability of owners to rent out their apartments.

Find a broker who is experienced in investment sales and buy an apartment with the widest appeal to renters. Two- and three-bedroom apartments in luxury buildings are usually in highest demand--with two bedrooms having an even broader appeal than three. A large one-bedroom that can be easily converted into two might seem like a good substitute for a true two-bedroom, but there are usually more of those available, so you'll have more competition. Having more than one bathroom is a major plus.