The challenges associated with a reaction change with the scale. Small reactions are easy to stir and cool, but highly sensitive to the environment , whereas larger reactions can take a lot of extra effort to purify. For the next little I’ll be focusing on small scale reactions, beginning with measuring reagents.
The gold standard is the serial dilution. In a 1/10 or 1/100 mixture, almost any compound can be accurately measured and added in a controlled fashion (in a 1mL syringe or gastight syringe, freehand or with a syringe pump). For less than 10mg of solids this is by far the best the preferred option, unless your material is extremely well behaved. However, making 100mM solutions is rather time consuming, and it can be a little irritating to constantly dissolve and concentrate one of your key reagents.
For stable liquids just dab a preweighed Pasteur pipette into your compound of choice. With a bit of work capillary action will draw up to 1.5cm of liquid into the pipette, which translates into about 30mg of material. Reweigh the pipette to get an exact mass, then either continue to dab or press your thumb against the top of the pipette to adjust the volume. When you have enough drain into your reaction vessel and rinse the pipette a few times by dabbing into the reaction solvent. 
For slightly larger quantities (50-800mg) a syringe comes in handy. Unlike a pipette a 1mL syringe won’t drip, and it’s relatively easy to rinse out into the reaction vessel. Neither method requires knowledge of the liquid’s density, though with the syringe I suppose you could determine it, if you chose.