I've been riding for about a year know and have my bike setup in a pretty upright position, but I've been thinking of making it lower/aggresive. I'm a bit apprehensive though due to hearing stories of sore backs and necks. I was just wanted to know if there are any precautions I should take before lowering it and any stretches to increase back flexability.

Before you change anything on your bike ask yourself whether you could be bending your elbows more.

You can experiment with the spacers on your stem - the worst that happens is that you don't like it and switch it back. Just go slow - one spacer at a time.

Drop it in increments of 0.5-1 cm at a time. Also try a 1 or 2 cm longer stem.

Few years ago I added aero bars to my roadie. Never experienced a sore back with them. I am on the aero bars probably 80% of the time. But like any other position recommend you get a good fit on your bike. May cost you a few coins but will help out with aches and pains.

Why not try it and see? You'll know if it's going to work or not long before you hurt anything.

Can you ride the drops for hours at a time?

I can get maybe an hour to an hour and a half. The main reason I'm asking is because I see other riders with a flatter back and then there's me sitting pretty relaxed.

The sore neck thing happens to everyone at first, after a while you get used to it. As far sore backs, I've never had a problem with it personally but I'm fairly flexible and still 'young' by BF standards.

A lot of people are suggesting to gradually change the stance, but I went from positive degree stem with riser bars to negative degree stem with 155 mm drops all at once. It was 'fun'.

Precautions? Don't cut your steerer tube and don't care what anyone else thinks a higher stem looks like. Because no one but you is really going to notice. Comfort is the key to happiness, not what a bike looks like around other guys in the parking lot before a group ride. Just experiment with spacers.

Noticed I keep my elbows pretty straight and locked up. I'll start to make sure I bend them but how much do I bend them? Is it just a comfort thing or should I do 90° like I see some do (mostly pros)?

As much as is comfortable.

Just do it...and don't put too much thought into it.

Do like that the other guys are saying. If your stem is flipped up...flip it down...start from there. Then go 5mm at a time with the spacers...go another 5mm every few rides. You should always ride with a slight bend in the elbows. It'll help absorb shock and you'll have better control over your bike.

This. No need to make any changes to your bike to start, just lower your position. Have someone take a video as you ride by and figure out where your head needs to be to get a flat back, then practice riding in that position and see how it goes.

You'll often be faster riding on the hoods with your forearms parallel to the ground rather than riding in the drops.

Here are some tips from Gerard Vroomen (Cervelo founder): Body position vs bar height – part 1