Well, naps are good... getting outside and taking mental breaks is good... and make sure you eat healthy, that helps with energy (or lack thereof)...
Also, dunno if it'll help, but there's a thread on ConceptArt with a ton of links for all kinds of art motivation issues, there's a lot of good stuff in there. It's this thread here: [link]I feel like if I create anything it must MUST be OMG PORTFOLIO WORTHY or I'm wasting my time.
I've felt like that, when I started freelancing especially, and even now sometimes when I'm in a long slump between jobs... Like I'm not sure what I should be working on, and I'm afraid to waste time working on "the wrong thing". It sometimes helps if I kind of set myself a schedule - work on work-related/portfolio/"important" stuff for a certain amount of time, then leave a little time to goof off and work on more random fun stuff. At least that way if I work on "more important" stuff first, I don't feel guilty about working on other stuff. (And often the "less important" stuff turns out to be worthwhile in the end, anyway.)
Though of course that means having time to work on anything to begin with... :/
Maybe if you lack time and energy, set yourself some itty-bitty daily art tasks for now? Maybe that would be a way to feel like you're accomplishing something, (however small), but without too much pressure.