My first thought would be a bad or partially broken bulb. Hit any hard bumps lately?
Any other lights flicker at all when you switch your headlights on, like your brake lights or dash lights? Does it improve it all or change behavior when you play with the headlight switch?
Because it is affecting both sides of your headlights, assuming the bulbs are good, I would then look at the fuse/relay for the headlamps followed by the headlight switch being the possible causes. A corroded or failing fuse/relay could display similar symptoms.