Aaarrrghh, geez, ask a simple question whydontcha? lol
Disclaimer: below is written with the implication of a new race for d20 D&D
To be honest, I base them all off of humans and change their physical description. Then I start adding little details, both pro and con, and I mean little. Even as much as a +2 bonus can totally unbalance something. Man, there is no real easy set rule here. Oddly, I find Pathfinders races to be the most balanced I have ever seen. Another way I look at it is this: what if everyone on the planet has an equal baseline for all stats, then, what little nudges can I do to add flavor to a unique race. If you end up with a race with all positive bonuses without negatives, you know need to make some level-adjustments. Making some bonuses situational or enviromental can alleviate that to some extent and bring the new race back to balance.
try this:
In cold enviroments: what races thrive, what races face hardship, assume human is avg.
In hot enviroments: see above
In forests: see above
In deserts:
In mountains:
In caves/underground:
In sunlight/darkness:
Then, look at:
What races are tough and resilient, what races are fragile, assume human is avg.
What races are fast, slow.
What races are smart/not so smart.
These lists can go on infinatum nearly. Make sure you apply each question to every race. Then compare bonuses and negatives. If one race seems to not have a balance, give it level-adjusting.
I feel like this isnt very helpful, sorry, just throwing this idea at you. Someone else can prolly give better rules/rationalizations here.
Maybe I will write out some of my own races tomorrow, its late now.