In 2009, Gerrit Coatzee published this elegantly simple soldering assistant. It is heavy and stable, and it rests on top of an SMD while you solder it. It also acts as a heat sink to protect the part. The device has never caught on, and that's a shame.
Part of the problem is that Gerrit made it the old way, by drilling holes in bits of wood like a caveman. So I've updated the design for the 21st century. Here is a printable gravity clamp to hold your surface mount parts.
A M5×45 socket head machine screw clips into the head, and three more M5 screws are used to provide ballast. You should sharpen the point with a file or grinding wheel (like a caveman!). In the photos, I'm soldering 5050 parts, so I used an unsharpened tip. Tips are interchangeable. Create a few different sizes and snap them in.
The OpenSCAD source file is attached. It should be easy to adapt to different screw sizes, overall dimensions, or head angle.