-80 or higher. (higher in this case would be -75)
and closer to cell site, antenna amp, or bigger antenna. would be the first 3 things to do.
Is that with or without a case?
There is an app that will show you a rough idea of where the cell you're attached to is located. I forget the name right now. Basically it will show you the cells you are within range of - and highlights the one you are connected though. on a map. so as to give you an idea of how close you are and what sort of coverage you are getting.
oh and who is your carrier