I have the same filter. According to Eheim they require cleaning once every 3 months, and a check over of the system and parts every 6, based on a call to a tech last week. (each system is different though, clean as needed)
How I 'clean' mine...
During my normal water change I take 2 5-gallon buckets and fill each a few inches from the top, so about 4 gallons full. Once every 3 months I pull out the filter and bring everything into the bathroom.
1st tip - watch the wet/dry filter and you can see it fill & empty, make sure to close the valves when the filter empties, before it starts to re-fill. That way you do not have to lift an extra couple gallons of water when you have to move the filter. For me I usually close the intake valve, then unplug it, then close the output valve, all quickly. I found if I just unplug it when itis empty it siphons and fills quickly again before I can close the valves.
I open the filter, remove the 3 media baskets and place them in the tub, with one of the buckets of water. I then 'dip' the baskets in the siphoned aquarium water until clean. Honestly have yet to get a lot of 'junk' from them. I am amazed at how clean they stay with out any mechanical filtration.
Then I use the other 5 gallon bucket and slowly pour water over the 3 media baskets to 'rise' them. Then I re-install them into the canister and set-it back up, works great.
Do NOT use any kind of fresh water to rise the media baskets!!!
2nd tip - make sure the baskets are perfectly level! I had one that was not sitting flush on the other, only off by 1/8" and the filter did not work properly, would not empty and fill, just stayed empty. When you put the 3rd basket in with the plastic cover, use your eyes and make sure the top basket is level with the plastic top edge. (it will not physically be level, a few inches down, but you can use it to sight it to see if it is level).
Following that system I have not had a problem and it works great. I am happy with the way it controls the system as well, levels are perfect!
Any other questions let me know!