I would start off with the invitations. Use ones in the shape of a gift basket, which will give your guests a little hint.
Then, plainly tell your guests in the invitation that you want them to bring smaller items to go in the basket, as opposed to big items like baby furniture.
Here's an example:
"We're having a baby shower for Crystal Smith!
Please join us on Saturday, August 16th
at the Guest of Honor's home:
123 East Main Street, Philadelphia, PA 19000
The celebration will start at 3 o'clock PM
In addition to your presence, please bring a gift donation for Crystal's gift basket!
Small baby items such as clothing, feeding and baby supplies are just what we need.
The more items we receive, the bigger her basket will be!