Current events makes this question even more poignant
Curiosity selfie has been released just a few days ago, showing better the collection of sand on top of the rover:
After snagging a new rock sample on August 9, 2018 (Sol 2137), NASA's Curiosity rover surveyed its surroundings on Mars, producing a 360-degree panorama of its current location on Vera Rubin Ridge. The scene is presented with a color adjustment that approximates white balancing, to resemble how the rocks and sand would appear under daytime lighting conditions on Earth. Two versions are included here: one with scale bars, and one without.
The panorama includes umber skies, darkened by a fading global dust storm. It also includes a rare view by the Mast Camera of the rover itself, revealing a thin layer of dust on Curiosity's deck. In the foreground is the rover's most recent drill target, named "Stoer" after a town in Scotland near where important discoveries about early life on Earth were made in lakebed sediments.
Edit: In light (or lack of light in this case) of the current dust storm on Mars discussed in the recent NASA news article Opportunity Hunkers Down During Dust Storm and since almost two years have passed, I've un-accepted @Phiteros' answer to see if there is an update on ExoMARS' dust-on-solar-panels strategy.
Since it does not have an RTG, ExoMARS relies on efficient collection of solar radiation for its mission to succeed.
above: "This global map of Mars shows a growing dust storm as of June 6, 2018. The map was produced by the Mars Color Imager (MARCI) camera on NASA's Mars Reconnaissance Orbiter spacecraft. The blue dot indicates the approximate location of Opportunity." From here. Credits: NASA/JPL-Caltech/MSSS
I remember reading stories about the Spirit and Opportunity Mars rovers related to solar power and dust - about problems with dust storms, and parking on an incline for the winter in order to collect enough power to 'stay alive'.
The Curiosity rover carries a large, powerful RTG for both thermal heat (circulating fluids) and of course electrical power.
Now it seems the ExoMARS rover will use solar panels again. The do seem a bit larger compared to the size (or volume) of the craft compared to Spirit and Opportunity, but I am wondering, what management techniques are begin considered for the ExoMARS rover to handle covering of the horizontal panels by dust, or low solar elevation in 'winter'?
above: "A 1:4 model of the ExoMars rover at the ESA pavilion, 2007 International Paris Air Show at the Le Bourget airport." From here.
above: "The deck of NASA's Mars Exploration Rover Spirit is so dusty that the rover almost blends into the dusty background in this image assembled from frames taken by the panoramic camera (Pancam) during the period from Spirit's Sol 1,355 through Sol 1,358 (Oct. 26-29, 2007)... Dust on the solar panels reduces the amount of electrical power the rover can generate from sunlight each sol." From here.