Since your feet should hardly touch the ground when the saddle is at optimal height, what is the best approach to stopping at a stoplight and then quickly re-starting when riding in traffic.
Since I exclusively ride cleated pedals, what I always do whenever I need to come to a full stop is to shift to a low enough gear (on flats I'd shift to 34/21 or 34/23 -- I have a 'compact', ie. 50/34 crank), unclip my left foot, brake, then as I come to a stop, I shift my body towards the top tube and stand over it with my left foot on the ground. Usually when I put my foot down the bike would still be moving very slowly, but friction between the cleat and tarmac and/or a touch of the brake would stop me immediately.
When I need to move again, I'd take the right crank arm to the 2 or 3 o'clock position, give the bike a forward push with my unclipped (left) foot while my clipped (right) foot simultaneously pedals. And as my right crank arm reaches the 6 o'clock position, I'd lift my left foot off the ground, move my butt from over the top tube to the saddle, clip my left foot back in, and continue riding.
These are the options I use depending on the circumstances:
- Trackstand: Requires a lot of practice and it is a bit of a swhowoff. (This is the one I use the less)
- Partially Dismount: Preferred when riding cleated pedals. Dominant foot stays on the pedal, and the pedal is kept ready for a full stroke (at 45 degrees over the horizontal as the other answer says). The other foot goes to the pavement. Preferred on busy crossings or when I have to use a lane not next to the curb.
- Right foot on the curb: My Right foot is the dominant one, but when I get to stop besides a curb, the fact that I remain in the saddle almost makes up for having to take the first stroke with the left.
- Use your hands: When there is a light pole or a suitable road sign, I keep mounted (and cleated) and use my hand to keep balance. Sometimes I can even put the shoulder against a pole or wall or the end of the handlebar. This one lets me re start fully engaged to pedals.
If you can't time it so you don't actually get caught at the light, you pretty much just have to dismount. As Daniel R Hicks mentioned, you can use a curb if one's available, but that's not always the case. And on a personal note, my strong leg is my right leg and since I live in America, the curbs are on the right, which makes that method less appealing to me.
If you set your pedals properly, dismounting can actually help you accelerate more quickly when you get going. Set the pedal for your stronger leg a bit less than 45 degrees from vertical. This allows you to get a full stroke right off the bat. When the light turns green, all you have to do is stand up, which puts your full weight on the pedal, and you're off.
If you're riding a geared bike, it's also a good idea to shift into the appropriate gear before you actually come to a stop. It takes a bit of practice and familiarity with your bike to know what gear that is, but it comes pretty naturally over time.
Well, you can't get around losing some/most of your momentum at a red light but if you do it clever, you can play with the timing of the lights and decrease your speed in such a way, that you won't have to stop completely.
If you're very familiar with who gets green after whom, you can plan even better.
Good practice is also to not put the feet down but balance or trackstand. That way, you can keep your feet ready on the pedals.
You can position yourself a bit behind the stop line (if possible) and take off on mid-orange, so you're already going when you pass the line at green.
The accelerating is common technique and power workout.
Keep your strong (first pushing) foot at 3 o'clock, don't lose time with a lazy second foot. Start in an appropriate gear (depending on road gradient and bike specs) and work on the perfect shifting time. Doing the shifts quickly will save you some more time.
And then: full thrust!