Most space bars have a piece of wire in them, to push down the bar evenly, however on older/cheaper keyboards, this bar will tend to come off.
Step 1: Find a paperclip
Bend these little sections into a v shape with a very small gap.
Step 2: Install
You can now reinstall your space bar with confidence!
Step 3: Done
Feel free to abuse the space bar with extra force, now that it's extra strong!