How to minimise solution concentration changes due to evaporation?
What if I stored some water in exactly the same conditions together with my sample, would I be able to approximate water loss in my sample from the water loss in this reference container, or does the difference in the concentration of the solute affect the rate of evaporation too much?


That would be a gross approximation, not suitable for a standard solution. Your solution and pure solvent do have different vapor pressures according to Raoult's law.


  What temperature?


It depends on solvent, solute, and their interaction. I'd check two main things: i) solubility at the storage temeprature, which must be as close as possible to the solubility at the working temperature; ii) vapor pressure of the solvent, that gives you a rough indication of the storage temperature. You could take a look at the temperature dependence of the solvent vapor pressure and solute solubility - or, if seriously in doubt, even try simulating the thermodynamics of your solution using a software. 


  What container should I choose?


Volumetric flasks are usually the best choice. They minimize evaporation compared to other glassware (bottles, beaker, etc.) and, most important, they have the most accurately placed graduation mark. Use the appropriate rubber cap and close accurately with laboratory film. PTFE band between cap and flask cone might also help preventing losses.

At times, a slight overpressure above the liquid can be desirable. In this case, a gas valve (or a quality glass faucet) will be placed on top of the flask and nitrogen will be flushed inside. The flask can be stored capped by the sealed valve. Also consider the opportunity of degassing your solution by placing sieves on the bottom of the flask or by bubbling nitrogen through the liquid. Finally, consider the need for a dehydrating agent in the case of a moisture-sensitive solution.

But, most of all, don't keep standard solutions for two years but replace them every ~6 months.