As I'm sure you all know, the key to minimising sim crossing crashes is situational awareness. Be absolutely sure where sim boundaries, and even more critically, sim corners are, at all times.
Avoid corners like the plague. Before starting a turn, or other manoeuvre, be sure you can complete it before any sim boundary. Slow down for the actual crossing, cross travelling straight and level, and dont try to manoeuvre until you are safely over. Dont cross at a very shallow angle, and of course, never travel along a sim boundary.
Ok..situational awareness. But how?
Firestorm is the answer. The firestorm minimap has a "show" option (right click it). Choose property lines and map textures.
Yaay! There are the sim boundaries! The regular big squares. Keep minimap open at all times. I have it bottom left, one quarter screen height. Zoom it with the mouse wheel to show you about half way into nearby sims.
Now watch it like a hawk, plan ahead, how you will tackle the next crossing.
And of course, don't try to enter a plain black one. Nothing is there.
I do still crash, of course, but not often. I make quite long journeys. Sailboats, submarines, and helicopters mostly. Speed kills.
And I carry my full load of attachments, scripts etc. I tried the low-lag classic outfit thing. Seems to make no difference, despite all the advice above.
Since I started this regime, although long journeys are still slightly challenging, it's mostly ban lines that get me (damn them).
Having to be alert, scanning continually across multiple information sources is actually quite realistic. In RL flying, you are taught a particular pattern of eye movements, scanning across the instruments, which soon becomes instinctive.