I've been working on some new techniques, including slapping and developing my technique with respect to double stops. I found a useful "masterclass" written by John Goldsby in several 2007 issues of Double Bassist magazine (now defunct and incorporated into the Strad). The Strad has been kind enough to make these articles available on its web site. The articles by John Goldsby are great. Lots of good ideas for developing a good double-stop techique. I've been doing scales using thirds and fifths double-stops (something suggested by Goldsby). While the articles focus on jazz playing, the ideas are equally applicable to other genres (classial and rockabilly).
Also included on the site is a masterclass by Owen Lee on the double bass solos in Mahler 1 and Prokofiev's Lieutenant Kije suite.