1) Warm up. And I mean really warm up - a sold 30min -60min and then hit some intervals so you are fully in tempo. 2) Find a nice long stretch where you can get yourself close to LTHR. Or, in otherwords, on the rivet. 3) Now then , when you are in-tempo, then slowly raise yourself by your core muscle group so you are as dynamically 'posed' on your saddle as possible. With only very light, to zero, pressure on the bars. 4) Here is the secret sauce, feel if you tend to creep forward or back on the saddle. If you tend to creep forward then you may wish to move your saddle a couple mm forward, and if you tend to creep back, then move it back. 5) What you are looking for is the 'anchor point' , where you have very stable position on the saddle in such a way you are not trying to move your self forward or backward on it. 6) This will very likely correspond with appx 145 degree leg extension and Knee Over Pedals about the spindle. But don't need to follow this rule exactly. It's just a sanity mark.
Key point is really learning that 'anchor point' and how it affects your biomechanical efficiency which is going to be closely tied to your metabolic efficiency as well as general riding form combined with the torque-rpm curve you are attempting.
Don't mistake poor form for driving you either forward or back! You need to be able to pedal evenly and with a level foot (or slight ankling). If your saddle is too high, for example, and you have poor technique, you'll tend to continually drive yourself off the saddle and always think your saddle is too far back! (very common mistake among junior triathletes)
Riders that have strong form will be able to expand their envelope of torque-rpm and utilize different muscle groups for different purpose, and thus will tend to move around on the saddle quite a bit to optimize their anchor point. Riders with lesser form (and experience) should just go through the steps above and find a happy medium.
Take exceptional care to NOT have your saddle too high! This will just apply too much pressure on your sit bones, generally unevenly, cause you to start raising your heel , and all sorts of mayhem. Just put saddle at the point where your hips don't rock, then go from there a few mm at best up or down. You should be able to spin at least 110-130rpm cadence without any bouncing or comfort issues.
Riders who find themselves doing more continual higher cadence, steady power sessions may find that being more forward is more efficient, while riders who like to punch out hills or variable terrain may find a more slightly rearward position favorable. Tri riders may sit even farther forward so that they decrease the effort on their calves, which they will need to draw heavily on for subsequent runs, so how you sit also depends on what you plan on doing later and after the ride and what primary muscle groups are affected.
Then you can easily take this one step further to adjust your stem, because once you are anchored, you know exactly where you will drop on the bars. Again, if you are positioning for Crit, TT, Tri, Touring, Climbing it will all be slightly different but you should be able to find a relatively neutral position that fits you best.
By the way - if you have an iPhone or iPad, there is a very cool app for checking your alignment on the bike. It's called Bike Fast Fit, all you need is a fixed trainer and someone (or a tripod or gaffer tape) to shoot yourself riding the bike. Nothing can really substitute a super custom fit, but if you just need to dial in to a decent zone, this app really helps and following a dynamic 'road test' like above. Besides, your 'fit' will change quite a bit as you ride or train until you have reached a normalized fitness state. And even then, you'd probably be tweaking it from time to time based on what you want to do (race, tour, sportif, commute, etc).
Sit bone measurement is easy. Sit on a chair with a piece of cardboard on it. Lift your legs and you will 'impress' it slightly. Then measure between the 2 depressions. Pelvic rotation - yeah, you'd probably need something with an inclinometer.
Get an easy impression by putting a cup or two of flour in a big enough baggie, smooth it out some, and put it on, say, a closed toilet lid, and sit on that. Rock forward so your torso is down, like elbows/forearms on your knees.
Get up carefully. (the flour will still be okay, but you should toss the baggie...)
That said, I think measuring sit bones is over-rated--it's a number, but it's no guarantee.
I'm a total amateur, but given what Tim has said, there may be something to be said for a brooks swallow. It's big/long enough that your sit bones might find their sweet spot on their own, and then what'd you'd need is a fit from Chuck to get its positioning right.
Also, as stated by others elsewhere, there's no one ideal position on a saddle--maybe half or the majority of time is in one place (or almost), but fore and aft of that is also usable and a nice change.
Both the Specialized Store (believe it moved to Shinjuku/Ogikubo area) and Nalshima have a fancy gel version of flour bag jdd mentioned. Basically it looks like an elongated '8' made out of gel that they drop two small ball bearings onto once you get up.