Dig in lots of home-made compost, which will help to retain moisture. You'll have to water your trees a lot in their first year.

Just to reinforce Spritzhenry's comment about watering - I planted a crataegus tree in my garden in May 2007. There were no flowers on it in 2008, but last year I was ecstatic to see buds appearing in late spring. However, I failed to water enough and although the tree survived this well and continued to be healthy, the poor buds shrivelled and died before they even opened. This year I have learned by my mistake, and have poured water on it at every opportunity, and am being rewarded by the beautiful bright pink buds that are just starting to open.