For pink hydrangeas add lime to the soil (will only work if there is no aluminum in the soil).
For blue hydrangeas add aluminum to the soil.
Turn your long-handled garden tool into a measuring stick for proper spacing of plants! Set a ruler or tape measure next to the tool handle, and using a permanent marker, write the inch & foot marks on the handle.
To have garden twine handy when you need it, just stick a ball of twine in a small clay pot, pull the end of the twine through the drainage hole, and set the pot upside down in the garden. You'll never go looking for twine again.
Here's an entire list specifically for ideas on what to do with all those garden catalogs - hint, hint!: don't throw them away!