Lower back rounding, wrist flexibility & getting that "locked in" feeling when unracking.
Your lower back rounding could be due to either the lack of upper body strength to support the weight or the weight being improperly front racked.
In the case of the former, I suggest doing a lot of rows (Pendlays is my favorite variation), weighted pullups, and deads.
In the case of the latter - well I guess I'll cover both issues regarding wrist flexibility and unracking the bar.
If you have terrible flexibility in the wrists, then I suggest doing the following stretches:
Put your hands together and roll them a la Wanderlei Silva in his prefight rituals
Wrist circles
Open and close your fist rapidly as if you're flicking something
Get a pvc pipe or a broomstick and grip the bar wide and do over-and-unders with them.
For static stretch I like to do the following:
Sit on a bench and place my palms on said bench with my fingers pointing at 6'clock. I gradually tilt my arms towards myself while continuing to put weight on the wrist to give it a nice stretch. I also do this with the uprights on a squat rack.
If you have a partner, then try this out: front rack the bar with all of your fingers wrapped around. Have your partner place his hands on your triceps and lift your elbows up. Hold that stretch for about 10-20 seconds. Do as many reps as you can.
To front rack the bar properly, first you need to grip it at the right area. For me, my pinky fingers are right inside the outermost rings (2nd ring). That's where I feel most comfortable because I have wide shoulders. Your hand placement may vary due to a different body structure. See what fits best through trial and error.
There is a groove alongside your clavicle. The bar will be set in that pocket and on top of your shoulders. At no point will your arms be supporting the weight - their only purpose is to keep the bar from falling off from your shoulders. It's hard to exactly tell you where this groove is but again, just find that area through trial and error - slap a pair of dimes or quarters onto the bar and practice front racking with them.
If it feels like the bar is choking you, then relax, that's completely normal. If at any point you feel like you're going to black out, then drop the bar. Hell, that's completely normal too. Just keep your back straight, all of your fingers wrapped around the bar, and your elbows up.
Here's another stretch that could be very beneficial to you: put 55-95 (or maybe 135, if you can handle it) front rack the bar with all of your fingers wrapped around it and squat. Pause at the bottom position for about 20-30 seconds while maintaining a straight back and keeping your elbows up. Do that for 3-5 reps.
Well, I've pretty much exhausted my knowledge regarding this subject. I'm gonna go have a late midnight snack.
Good luck, and happy lifting SexyMahnaz!