If you had real drywall, an alternative to a toggle or molly screw would be to drywall anchors; there are two types.
One type is fairly common (and might be able to be used with plaster - maybe) which has a plastic insert you install into a drilled hole, then when you thread the screw into it, it expands to wedge itself in the hole. IIRC, the largest of these can hold about 30-50 lbs in drywall (probably less in plaster).
The second type uses a large threaded insert; the hole you drill for this is typically larger than the screw which threads into it, but the anchor can hold much more weight (up to 150 lbs per anchor in drywall). I don't think they can be used in plaster, though, as they are meant to thread into the drywall.
I've used both successfully for a number of projects in my house; I have an entire wall (about 12 x 8 feet) of bookshelves (using shelving standards and melamine shelving) held up by the threaded anchors (hundreds of pounds of books). I have used the expanding anchors for heavy items like large pictures and mirrors, among other items.
The major downside with any of these anchors, though, just like molly or toggle bolts they leave larger holes which need to be filled if you move the item. Hence, you typically only use them for items which will rarely, if ever, be moved.
For drywall, for lighter weights, let's say pictures, mirrors, not book shelves, there are actually self-drilling and self-threading anchors, you don't need a drill, only a power screwdriver, there are both metal and nylon ones:
One type is fairly common (and might be able to be used with plaster - maybe) which has a plastic insert you install into a drilled hole, then when you thread the screw into it, it expands to wedge itself in the hole. IIRC, the largest of these can hold about 30-50 lbs in drywall (probably less in plaster).
The second type uses a large threaded insert; the hole you drill for this is typically larger than the screw which threads into it, but the anchor can hold much more weight (up to 150 lbs per anchor in drywall). I don't think they can be used in plaster, though, as they are meant to thread into the drywall.
I've used both successfully for a number of projects in my house; I have an entire wall (about 12 x 8 feet) of bookshelves (using shelving standards and melamine shelving) held up by the threaded anchors (hundreds of pounds of books). I have used the expanding anchors for heavy items like large pictures and mirrors, among other items.
The major downside with any of these anchors, though, just like molly or toggle bolts they leave larger holes which need to be filled if you move the item. Hence, you typically only use them for items which will rarely, if ever, be moved.