In the Advanced panel of your Network Preferences ensure that your network is listed at the top of the Preferred Networks panel
Here are some other resources:
WiFi update 10.7.3
Apple DNS Servers:
Move the Base Station If interference is particularly bad in one part of your house or office, move the base station to another location. But this can be tricky if your AirPort base station has to connect to the broadband feed coming into the house and that feed happens to enter at a particularly interference-prone spot.
Use the 5GHz Band Compared with the 2.4GHz band, the 5GHz band is practically empty. The newest AirPort Extreme Base Station is one of the few Wi-Fi base stations designed for home use that work in that 5GHz band. All of Apple’s Core 2 Duo laptops and iMacs (except the discontinued 17-inch 1.87GHz model) can also use the 5GHz band; with the build-to-order wireless option, so can the Mac Pro.
To switch to the 5GHz band, launch AirPort Utility, select your base station, and switch to Manual Setup. Click on the AirPort button and select the Wireless tab. Select 802.11n Only (5 GHz) from the Radio Mode pop-up menu. Click on the Wireless Options button and select Use Wide Channels. Click on Update to restart the base station.
Currently, no Mac-compatible adapters enable you to use the 5GHz band in older Macs or with older versions of OS X.
These Firmware updates may or may not apply to your system, but may help with your particular problem.
On the Italian site, I got instructions directly from Apple.
They told me to go to settings (Preferenze di sistema in my Italian system)
Something apears with WiFi indicated telling you whether or not your WiFi is connected.
Go to Advanced (that's probably what it is in English).
Click on TCP/IP.
On the right, I found a drop-down menu which led me to "renew"? "refresh" (mine is in Italian) assigned DHCP.
Trembling, I clicked the fatal button.
Apple's help sheet then sends you into a whole set of other things to do, but I stopped when this refresh (I tried a restart) seemed to have solved my problem.
Try that for now, and keep a copy of the MAC instructions on your desktop (File/Save page...etc.,)
If that doesn't solve the problem, go back to your saved Apple help-sheet and continue to follow their instructions, which involve the "portachiavi" (keychain?) function.
Yes you can.
I own an iMac and run it with OS X 10.7.5. Ever since system update 10.7.3 (February 2012) my mac more often than not does not connect to wifi on boot. I have to select diagnostics and connect manually. I did a design of experiment and have now discovered that I do not have this problem if I wait about a minute to log in after the login prompt displays. I like my mac for it's fast startup. It is a shame to have to wait unecessarily for it to connect to wifi before logging in. I kenw this was a problem with the system update and this confirms it. Does anyone know how to prevent this from happening so I don't have to wait?
The below has solved my Safari hanging when loading web pages and eliminated my finding the wifi on boot up. It has been a month now and waiting a minute always works for connecting to my airport extreme on boot. I was cleaning behind my desk and realized that the ethernet cable was still attached to the back of the iMac even though I had moved the airport upstairs. I disconnected it. I also opened network preferences. The wifi was moved a long time ago to the top for priority, but the inactive ethernet and firewire were still listed. I removed both the ethernet and firewire so the only connection listed is wifi. My computer has now reliabily found the internet for the last 3 weeks without waiting. It appears that having the other two options disrupts the connection process on boot. It also must cause the computer to continue trying to use the connection when using Safari.