How can I improve water volume with a half-inch supply?
There are 2 possibilities to correct this problem unless, as described in previous answers, there is a restriction somewhere in the piping, bad valve, plugged line, etc. If the problem is not one of these then you need to raise the incoming pressure, if possible or increase the size of the incoming pipe.  

To increase the size of the piping, first check to see what is the size of the incoming piping from the street. If it is a size larger than the 1/2" you have in the house, then I would change the size of the piping to 3/4" from the larger street size all the way to the water heater. By doing this you are supplying both the hot and cold water supplies with it's own 1/2" supply line. Any take off feed between the incoming water and the water heater would be by a 3/4"X3/4"X1/2" reducing tee. 

I do not know what the codes are in your area so you may be able to do it yourself or you may need to have a registered plumber do the work. 

I would not use "shark bite" type fittings since they have some restriction built into them. If you have a 1/2" water meter, it will not add much restriction due to its  short length. 

To raise the pressure you could adjust the PRV in the supply line if you have one. If you don't have one then you are limited to the pressure in the supply line.