Optimizing magnetic field force of a coil
As mentioned in another answer, the magnetic field strength is proportional to amps and turns. However, the current is dictated by drive voltage and coil inductive reactance. Given that inductance (for a tightly wound coil) increases with turns squared it's usually counter-productive to add turns. A simple example is doubling the turns - the inductance rises 4 times and the current therefore falls 4 times and the net effect is that ampere-turns have reduced by two.


  As I understand it, adding turns will increase field strength, but
  adding turns will increase the coil's resistance and therefore reduce
  current.


If your coil design is at the point of causing worries about coil resistance then it's not a good design - the dominant impedance should be reactive as implied above.

There is always an optimal value but this depends on several factors that you haven't really told us about: -


Oscillator design
Tuning regime
Current available for the oscillator/coil
Coil design/shape etc..