millelect,
   I'll get this question in first I'm sure the other guys will ask it - more data required.
      * type of motor shunt, series compound
      * existing controls if any
      * supply source

  I would suggest it is compound and some rewinding will be required It may be that you could gain an edge if you talk to one of the specialist drive companies (eg Reliance).
  Will go and drag out my reference books but I have the feeling that someone will know the reference before I find it.

Regards Don

There are some physical limitations with respect to increasing torque output of a motor (AC or DC). Basically, magnetic flux=torque in any motor. The factors affecting flux are the current (ampere-turns) and the magnetic circuit formed by the "iron" of the motor in question. For DC, decreasing the resistance of the circuit (field or armature) by rewinding will increase the current (and flux). However, the result is not linear because the iron will become magnetically saturated at some point (true for any motor).

With respect to armatures, the amount of iron present in a design is normally the minimum amount due primarily to inertia considerations. For field frames, it is limited by frame type and economics (ie. once you have enough iron for a given torque output in a certain frame type, more iron costs more money with no benefit).    

Another consideration is purely physical. Assume that iron saturation is not an issue. For fields, there may (or may not) be enough physical space to accomodate a larger wire size. For armatures, the slot size is almost always no larger than that required to accomodate the necessary wire and insulation for the original design. As well, the commutator riser is sized for the designed wire. Sometimes you can get away with larger wire, but usually it is a compromise (if it is possible at all).

Next, specific to DC, the brush grade will need to be changed to maintain proper commutation with the increased armature current. Also, (with any motor), cooling will be an issue.

From a performance standpoint, increasing torque by increasing field current will decrease the rpm at original rated armature current. Increasing armature current will increase rpm at original rated field current. To change the rating of the motor while maintaining nameplate rpm will require a proportional increase in both.

Suggestion: Reference:
1. Slemon G. R. "Magnetoelectric Devices Transducers, Transformers, and Machines," John Wiley and Sons, Inc., 1966
A. Series Motor. The torque is controlled by a variable resistor in series with the rest of the circuit. Similarly, the series motor with compensating winding.
B. Shunt-connected motor has a variable resistor in the field winding for possible control of torque and a variable resistor in the armature circuit for starting purposes and for possible torque control.
C. Separately excited motor has its torque controlled via a variable resistor in the field circuit  or a voltage amplifier in the field circuit.

The previous post correctly identifies (somewhat dated) control methods for DC motors by type. Variable resistors were used with fixed power supplies, with the resistor used to reduce (trim) the voltage to the motor (hence the name "trim resistor"). Most power supplies had a voltage output greater than the motor nameplate, so it was (is) possible to trim the voltage to a value higher value than the motor nameplate. Resulting higher current = higher flux = higher torque.  

However, using trim resistors to increase torque beyond nameplate values will overload the motor. Most motors can withstand some degree of overload continuously depending on the robustness of the original design, with older motors (like anything else) built 'stronger' than newer designs. With experimentation, you may find a level of overload which does not result in unacceptable temperature rise or commutation problems (maybe 5%, maybe 20%, maybe none). However, the life of the motor will be shortened, perhaps dramatically.