If you DM is feeling generous, maybe you could convince him to house rule a "Still Spell-Like Ability" feat (since most spell-like abilities don't have somatic components, this is a hole that needs to be filled for warlocks anyhow). This would allow you 3x/day stillness, but it would have to be keyed to a specific invocation (as opposed to 1x/day, not keyed, for sudden still).