1.) Turn your phone to Flight Mode then back again
Apparently your phone can sometimes be holding onto an inferior signal from a mast, but by turning it onto flight mode and back, it will connect to a fresh mast giving you an improved signal.
2.) Go upstairs
Getting a clean ‘line of sight to the mast is the best way to increase your signal, so sometimes the best way really is to just go up a floor.
3.) Watch your hand, and phone case
In older models, your hand or phone case might be getting in the way of the aerial. Remove the case if you want increased signal, and watch how you are holding your phone. You could always add a wireless headset to minimise disruption.
4.) Check your blackspots
Most properties have areas where the signal will be weaker. Check your house and office for areas where this is so you can make sure you place your phone where the signal is strongest.
5.) Open a window (or just go outside)
Some windows have insulating panes that can interfere with mobile signal. Eradicate this problem by opening them, or just go outside if you have a balcony or terrace.