The humble Rubber Band. It makes your bike a fairly rigid plain allowing you to easily lean your bike almost anywhere without it toppling over.
This may not be too effective with a very heavy handlebar bag or low riders
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk