Here is my setup:
∗ LaserJet 2100 - bought this refurb for about $80, including refilled cartridge
∗ LaserJer has ethernet module (otherwise it's LPT only, difficult to drive in this day and age)
∗ Drafting paper
∗ Set the density to 5 (default is 3, tried to do this via the new Universal Printer driver and failed, then did it by sending the specific escape codes via lpr to the network port where I have attached it)
I used to print two copies on the drafting paper and align and glue them perfectly. Worked well. Then I read about and bought a toner density spray. It's technically designed for transparencies, but works very well with the drafting printouts too.
I now only use only one copy. Blackness is perfect, going down to 12 mil traces every time and under than (e.g. for 0.65mm smd parts) with some care.
The critical part of my process now is getting a uniform thick coating (I use Positiv 20 on bare stock). Exposure and black opacity is no longer causing any troubles.