Here's a list of things that have worked for some folks. New poster's should try these step's to try to fix their issue:
-Reset SMC. (http://support.apple.com/kb/ht3964)
-Backup and then delete the files in /Library/Preferences/SystemConfiguration/ directory. Reboot your machine after this. This will delete all network preference's.
-Check if you have the firewall enabled. Disable it. Delete this file: /Library/Preferences/com.apple.alf.plist Reboot your machine.
-Check if you have any kind of antivirus installed. eg. Avast etc. Uninstall it.
-Check your DNS settings. Use a known good DNS like 8.8.8.8 and 8.8.4.4
-If you have multiple Location's set up in Network, try deleting them and only picking Automatic.
-When your connectivity drop's, ping the router and see if that work's. The issue might be with the ISP.
-On your wireless router, pick a cleaner channel. If using 2.4 Ghz, use HT20 only.
-Pick WPA2-Personal and your preferred security. If that does not work test if Open/No Security works.
- If you are using a 5 Ghz channel, pick ch 36/40/44/48 and try if it works.
- Make sure the beacon interval on your Access Point is set to 100ms.
-Make sure the Mac see's the AP at a good signal strength. -35 to -70 is a good range. Hold down the Option key and click on the WiFI Menu Extra for this info.