My school recently bought an £800 self build 3d printer from bits-from-bytes which works in a similar way to the rep-rap printer.
As such, it had many similar problems mainly models shrinking,collapsing or deforming fromuneven cooling of the part.
We researched the problem and found that one of the most common solutions is the hated bed. this is how we did it.

Step 1: Options

1.pcb heating plate*
2.metal holding blocks for ceramic rod heaters*
3.pre heating with a hot-air gun*
4.(the option that we chose) electrical resistors underneath the bed.

Aditionally, a temperature box helps. (next project?) .

*we had none of these parts at school, so i have used gogle images to find them (no copyright infringement intended) if the original photographers want them taken down, i will make it so.

Step 2: The Science Bit

When the resistors are linked in series, it totals 6r resistance.
at 12 v, we can use the equation V=IR to fint that it is at 2 amps
this essentially creates a 24 watt heater beneath the build bed.(P=VI)

Step 3: Parts

We used:
4x 1.5r 50W electrical resistors (the electronic ones would burn out)
1x Aluminium plate (200X200X5mm (LxWxD))
8x Countersunk head M3 Machine screws.
Kapton tape
a 12v power supply

Tools
M3 Tap
drill press with 4mm Drill bit
Countersink
ruler
centrepunch and hammer.
paper/pvaglue/pencil or engineers blue and scriber

Step 4: Marking Out

If using paper, glue (lightly and with water-based glue) the paper to the surface. If using engineers blue, apply to the surface and leave to dry.

1.Draw an X across the aluminium platecorner to corner to find the centre.
2.Along these lines, find the mid point (roughly) between the centre and the edge , and mark a perpendicular line (i used 5cm, but any sensible length will work so ling as all the resistors are equidistant from the centre
3place the resistors on the bed (centered on the 'X' lines) and mark the mounting holes
4 centre puch the hole marks

Step 5: Building It

1. drill and counter sink the holes that you marked on the aluminium plate
2. Tap the mounting holes of the resistors
3. use the countersunk screws th attatch the resistors to the plate
3. solder resistors in a series circuit
4. Attatch heavy guage hook-up wires and test- make a note of temperature (don't exceed the melting point of your chosen filament 70C is probably about right
5. attach to the printer
6 use your printer as usual, but better.