If you use a trunk bag (or rackpack or racktop) over your rear panniers like us, you will find that it sometimes needs to be fastened down with more than the built in clips. We have found toe straps (from pedals) to be the most secure way of keeping our bag sitting tight – especially on bumpy roads.
Many others use octopus (occy) straps to secure their load, but we've found toe straps to be smaller, lighter and make it easier and quicker to take the bag off.
We connect the toe strap just under the top tube / seat tube junction and to a handle on our Ortlieb Rackpack 49 bag. You can then tuck the excess part of the strap through the gap.
Once it is properly secure, there is not much which is going to allow the rackpack to slip backwards!