It is true that many packets sent over WiFi be collected using [ARP spoofing][1]. Because ARP operates on the link layer of the OSI model, there is very little you can do about this.

What is currently accepted as best practice regarding WiFi, is

 1. Use a properly designed security mechanism: [WPA2][2], and
 2. Use a strong shared key: 15 characters at minimum **and** randomly generated.

The password _must_ be random because a single handshake packet pair (challenge and response) can be used to crack the key off-line by using brute-force, dictionary, and/or rainbow table attack (or a combination of these methods).

  [1]: http://en.wikipedia.org/wiki/ARP_spoofing
  [2]: http://en.wikipedia.org/wiki/IEEE_802.11i-2004