We always try to install blocking whenever possible, when the wall is gutted.
A spot of liquid nails (or silicone) under the mounting plate can help.
Plastic sleeves are pretty reliable if you get the hole just tight enough and use the proper size screws.
Casey

Interesting. Almost every time I'm working through some punch list, I'm dealing with those loose plastic sleeves.

Although I wouldn't use an EX anchor for a TP holder, these things look like they might hold up better since they spread out behind the wall.

http://www.mrgrabbar.com/SnapSkru-SP-Drywall-Anchor-Regular-3-8-5-8-p/30011.htm

...meant to say "EZ anchor" (not EX anchor)

Blocking is always the best choice, but for retrofit work where I can't fasten everything into wood, I've found that EZ-Ancor products work quite well, even for something subjected to continuing force/stress -- like a toilet paper holder. I've had the steel "Stud-Solver" product securing a TP holder for 10 years with no loosening. And the anchor is easy to remove and patch over if you ever need to do that.

Here is a link that might be useful: EZ-Ancor

I like toggle bolts for repair after the original has failed. The hole are already large so they work well.

Blocking is prefered, but Molly bolts are the next choice.

Plastic sleeve anchors that expand are not really suitable for drywall. The pressure of the expanding sleeve simply fractures the gypsum core.

The large screw thread plastic anchors for drywall are a little better, but are really for light duty (smaller pictures and such) and not something that is touched and used on a daily basis (like a TP holder, sink cup holder, etc).

The leverage of the item protruding from the wall WILL eventually cause the anchor to come loose.

While many of the anchors have decent shear ratings, the withdrawal ratings of most are poor.
Molly bolts expand the area the load is applied to behind the drywall and have a much better withdrawal rating.

If you yank a Molly bolt out you will make a hole at least an inch across in the drywall.

If you use Molly bolts enough there is a very nice setting tool that expands them by pulling the screw instead of turning the screw.

The setting tool is usually under $20 if you can find one.

The setting tool is almost required if you are using molly bolts in plaster.
The little 'teeth' designed to limit rotation while tightening the screw do not work in plaster or hard material.

I'm skeptical about the wall dog, but might be worth a try:

http://www.popularmechanics.com/home/reviews/4224987

"I'm skeptical about the wall dog, but might be worth a try: "

I have used them to hold shelving that supports itself tight against the wall, and to secure bookcases against tipping, and to fasten smaller EMT conduit to walls made with two layers of 5/8 drywall (sound walls).
They are VERY strong when driven into over an inch if drywall.

There heads are rather large and you need to take that into account when using them.

They are also really a single use product.
If you remove the screw and them use the same hole it will NOT be as strong as it was the first time.

I still would not use them for something that is 'touched' frequently, like a towel bar, TP holder, etc.

http://www.doityourself.com/stry/hangheavymirror

Here's a link that describes various hardware to mount items on walls.