What are some methods to achieve constant LED brightness over large temperature ranges?
Your original question mentioned a sensor, which now seems to be gone. How precise do you want to keep that luminosity constant? If it's for visual it's pointless: your eye's sensitivity follows a logarithmic curve, so that it can cope with both 1 lux moonlight and 100 000 lux sunlight. Therefore it is not very sensitive to small changes in luminosity.  

Most LED manufacturers will give a linear relationship between current and luminosity, so to get the latter constant you only have to keep current constant.  

  

Note however that you have to derate current at higher temperatures: 

  

So for this particular LED you can't go higher than 5mA if you want the same luminosity for 25°C and 85°C.

The Supertex CL25 is a temperature compensated constant current source for LEDs.

  

This has a 0.01%/°C typical temperature coefficient. It is 25mA however, so you'll have to use a LED which doesn't need to be derated below that.

Rocketmagnet makes an important remark: "It may be the phototransistor which is sensitive to temperature." (RM, I hope you don't mind that I copy it here.)
Indeed, it's no use to have a temperature controlled LED driver if the sensor's reading varies highly with temperature. You'll have to look into that too.