My daughter has thick and very long hair (to her fanny). Brushing is generally a pleasant experience (now brushing her teeth makes me want to pull my hair out!!). I'll share a couple of tips with you.
HAIR:
Definitely use conditioner on her hair after shampooing. Apply in one direction only...from the crown to the ends of the hair. Don't rub it back and forth as it will make additional snarls.
I always make sure to brush from the bottom up, in small segments. You still brush in a downward motion, but you start at the bottom couple of inches...get all the snarls out there, then move up a few inches and repeat until you've reached the top of her head.
Make sure to hold the upper hair tightly in your fist. It minimizes the pulling and tugging that hurts her scalp.
Try a distraction, like using this time as allowable tv viewing. Use it as a reward (I will only leave the tv on IF you are behaving and sitting still enough for me to finish.)
Follow up with praise and sometimes a treat. (You know, you did a good job with hairbrushing all week! I think I'll give you a sticker! [or other reward]).
If she takes everything out of her hair, try braids. Put the elastics in really tight so it isn't easy for her to remove them. And it takes a lot of work to undo a braid, so maybe she'll just give up and get interested in something else. Besides, braids minimize the movement of the individual hair shafts so the result is less snarls. :-)
NAILS:
Wait until she's asleep (like in the car, just before you take her out...or taking a nap at home).
Have a "girl's salon day." So the Satin Hands (Mary Kay) or similar system. Talk about how great your skin feels afterward. Show her how to use the emery board. Buff your nails till they shine. Just have fun "doing" your hands/feet/nails. Make it look really fun or like a special mom-daughter time that she will look forward to (and you'll get fresh nails, too!).
Reward her cooperation by allowing her to get her finger or toenails painted if she behaves and allows you to finish the trimming.
Buy nail art appliques and use as a reward.
Get different colors of paint and create your own designs (flags are easy and fun, flowers, faces, etc). Her hands may be too small to do much detail, but you can always do two tones or paint fun stripes or polka dots.
I hope at least one of those helps.
D.