What is the best way of fixing paintings into mounts when the actual painting is smaller than the picture frame so can't just rest in it behind the mount?

Hi Von! Have you considered using a double mount? That is using a second mount which can then be cut to accomodate your painting. I think double mounts add a great deal to paintings, and should enhance the picture which you're framing.
If I've misunderstood your question and the answer above makes no sense to you, then come back to me and lets try to find a solution for you.

Very best wishes,

Steve Williams
PA & RSC Wigan / Warrington / St Helens

Thanks Steve but I actually meant to the back of the mount irrespective of whether it is a double or single one ie do you use masking tape or something else?

Hi Von! Thank you for your clarification. The method I use, and probably most other artist and framers, is to use a Tab Gun. This is a kind of hand held stapling tool which on squeezing a "trigger" shoots a small, flat and pointed metal tab into the side of the frame, at the back.
Most suppliers of commercial picture and print sellers use this method also.
The way you would use the Tab Gun is to lay the frame face down, with the glass in place (if its a watercolour), then your Mounted painting, and then the Board backing.
The Tab Gun is then placed in contact with the backing board and the tabs shot into the frame at board level, at even intervals around the frame.
The tabs are flexible, so at a future date, they can be bent upwards at right angles and the board and painting lifted out of the frame.

There is an excellent value Tab Gun in the SAA Catalogue - check in the index, or on-line.

Good luck and happy framing!

Very best wishes,

Steve Williams
PA & RSC Wigan / Warrington / St Helens

Thanks for the advice Steve and for getting back to me so quickly.