In the halter of ignorance the truants pace,
Although having no use for one myself, it was interesting to come across The Instant Muse Poetry Generator. It gives you a first line to conjure with if you're having trouble being inspired.
I can afford the luxury of avoiding exercises that don't come from a natural curiosity or exploration for either the subject or the language. Warming up exercises are surely for people who sit there twiddling their pencils; not those who have produced pages of ideas in the time alotted.
I tend to work from either titles or subjects that I turn into titles. I haven't built up from lines or couplets for some while, although I do cannibalise old works and construct new ones therefrom. There does seem to be some block on composing on the back of someone else's words as I've still come up empty handed with Dave Downey's 'Oscar the oscillating otter' and that was years ago.