For closed networks (like home users and organizations) the by far most common way is to configure access restrictions in the access points. Those restrictions may include encryption and checks on MAC address.
For commercial providers, hotspots and large organizations, the preferred solution is often to have an open, unencrypted but completely isolated wireless network. The users will at first have no access to the internet nor to any local network resources. Commercial providers usually forward all web traffic to a captive portal which provides for payment and/or authorization. Another solution is to require the users to connect securely to a privileged network using VPN.
Wireless networks are little more secure than wired ones; in many offices intruders can easily visit and hook up their own computer to the wired network without problems, gaining access to the network, and it's also often possible for remote intruders to gain access to the network through backdoors like Back Orifice. One general solution may be end-to-end encryption, with independent authentication on all resources that shouldn't be available to the public.