I recommend before planting anything to work on the health of your soil.  Clay and Sandy soils can both be improved by applying a very thick surface mulch of manure.  The best time to apply the mulch is in Autumn - although it can be done at any time of year.  Leave the manure on the top of the soil - and let the worms do the hard work of improving clay soil.  The surface mulch will suppress weeds and give you a soil with a more open structure and by feeding the soil with nutrients - most of your plants will thrive.

I recommend leaving the mulch on the top of the soil for a season - then apply another layer and plant in the following spring/autumn.