I normally scrape off the high points of the artex with an old cheap trowel untill its as flat as i can get it.
Then make sure that the artex has not been aplied on top of lining paper :

I try not to bond out ceilings, as it adds a helluva lot of weight to a ceiling that someone else has artexed on.
If it needs bonded out, then i normally either rip it down and put up a new ceiling, or just lower the ceiling.
But if its just a fine stipple or something, i scrape it down as flat as i can get it, then PVA it a couple o times.
Then hit it with Multi.
Sometimes after i have taken the lines out the second coat, and flattened it a bit, i will mix a little bit on the hawk to fill in any pits etc....This can make a world of difference.
If the artex has been put on top of lining paper, then either re new the ceiling (Gyproc) or you might get away with just plastering it, but with no gaurentee
