Maybe this has been suggested before, but I thought it won't hurt to share.
While I was using wifi on my phone at home, I noticed that my mobile data allowance was getting used, even though my phone was connected to the wifi.. turns out, i had a patchy wifi connection at the time. And, before i knew it.. more than 100MB got used in a matter of 2-3 mins.. while i was downloading some phone updates..
So, my tip is turn off mobile data completely, when you are connected to wifi.. to avoid illusion sometimes that we're being connected to wifi, while we may not notice that the wifi signal has been lost, or there was some error... as the phone just reverts back automatically to using mobile data..
Save on mobile data, save money