When you have a few silos, it's best to keep an eye on them.
If it looks like they are starting to get full, and you just can't build those
Mammoths fast enough, here's a way to keep from needing more silos.
Start building an expensive structure (War Factories and such in C&C,
and Chonosperes and missle silos on RA) and when it finishes, cancel
construction. You will get all you money back plus have more room in
silos. (Tip: make sure there is no action going on so you don't have to
be distracted during a battle, I hate when that happens)