Approach grafting as a technique can be applied to bonsai trees either when needing to fill a few gaps in the surface footage, also known as nebari or when needing to increase branch density in the canopy of the tree.
In either case, once the graft is completed it is of paramount importance that there is no movement. There are several ways to prevent disturbing the graft but the easiest and quickest is to use tacks of some sort, which are then nailed into the tree.
These tacks, which I buy when I am in Japan are ideal for the purpose.
They have a rubber stopper which prevents you from driving the tack too far into the tree and also makes it easy to remove the tack using some pliers, after the graft is successful.
Their length ensures that even large grafts can be anchored but should only a short pin be necessary then it is a simple snip with a wire cutter to shorten them.
Although the pin is very thin which means they make a very small hole in the tree, plus are easy to hammer in with a panel hammer they are extremely strong and will not easily bend.