I had to cancel my cover competition.  The last minute rush I had hoped for never materialised, and without entries there is no contest.  It was really disappointing.  I'd expended a lot of time, effort and money promoting it only to receive less than a handful of entries.  In fact, I received a better response to my post on a Kindle forum announcing the flop.  Initially, I was inclined to think it was a total waste, but I'm beginning to see there are things I can learn from it.
My advice to anyone thinking of running a competition is this: 
1.  It's better to offer one larger prize than three small ones.  Even though there are many people offering their design services for free on writers' forums and there is no fee to enter, the bigger the prize the better the response is liable to be.
2.  Make sure your terms and conditions include something like: 'The organisers reserve the right to cancel the competition in the event of unforeseen circumstances.'
3.  Think carefully whether holding a contest is really likely to be worth the effort, especially before spending money on posters or advertising.
4.  Whatever you do, be prepared for criticism.
I would like to thank the few people who did enter and wish them well.  For now, I intend to withdraw from issues about covers and marketing, and to retreat to a place where the inhabitants can't criticise me - Najarind.  When the next Exiles of Ondd novel is ready to be published, I'll take another look at the cover of Discord's Child.