I had some trouble with the wifi connection on my Raspberry PI. The wifi connection died after a few hours of inactivity. I fond this script that solved the problem:

#!/usr/bin/perl
# NAME      wifi-check.pl
# AUTHOR    Kronalias
# PROVIDES  Checks to see if WiFi has a network IP and if not restarts WiFi
#
# CRONTAB   Run wifi-check.pl every minute
#           */1 * * * * /home/pi/my/scripts/wifi-check.pl >> /mnt/ramdisk/wifi-check.log
#
# HISTORY
# 1.01 - Base version
my $name = 'wifi-check.pl';
my $version = 1.01;

use strict;
use warnings;

my $wlan = 'wlan0';

use POSIX "strftime";
print strftime("%H:%M:%S %a %d %b %Y", localtime(time())) . ' ' . $name . ' ' . $version . ' ';

if (`/sbin/ifconfig wlan0 | /bin/grep "inet addr:"`) {
  print "Up\n";
} else {
  print "Down\n";
    `/usr/bin/sudo /sbin/ifdown $wlan`;
    sleep 5;
    `/usr/bin/sudo /sbin/ifup --force $wlan`;
}

exit;

Source: https://gist.github.com/roflmao/5293963 You may want to have a look at part 2 also.