I read this tip several years ago in a quilting magazine. I would give credit if I could find the article again, but I can't locate it. I am a pin baster and have tried several techniques for keeping my quilt sandwiches "flat" while pinning. This one works best for me.
All you need (in addition to your fabrics, batting and safety pins) are a carpeted area and some metal " T " pins.
(Note that the instructions are first, then the photo.)
First, lay your backing fabric on the floor, face down. Smooth out any wrinkles.
Next, secure the fabric into the carpet with a T-pin. Place the pin at a slight angle so it really digs in to the the carpet (and any padding below).
Repeat down all sides of the backing fabric, with your T-pins about 18" apart. Smooth the fabric as you go and re-position pins as necessary once they are all in place.
Once the backing fabric is securely pinned in place, lay the batting over the top. Smooth flat. Unpin one of the pins and replace, this time going through both layers (batting and backing). Repeat for each pin, taking care to keep both layers flat as you go.
Once all the pins are re-secured through both layers, lay the quilt top over the batting (right side up) and repeat the pin replacement process. This time, you'll be pinning through all 3 layers (top, batting, backing).
Let the Journey of a Thousand pins begin, as you pin baste through all your layers as usual.
Once all your safety pins are in place, remove the T-Pins from around the edge of the quilt. Gently lift your new quilt sandwich off the floor. If any pins are too attached to the carpet (see photo below), you can unhook and re-pin that pin (minus the carpeting). You'll get a feel for where the carpet it pretty quickly and I usually only get 1 or 2 of these "attached to the carpet" pins per quilt.
Quilt as desired.
~*~ May your day be full of Love and Ladybug Hugs ~*~