what to do after topsoil

You need to dig in as much organic material as you can, including mushroom compost, home-made compost, well-rotted manure and also some horticultural grit. It should be dug in, not just put on the top - hard work, I know, but it'll pay dividends in the future. Also - whenever you plant anything, make the hole bigger than normal, and back-fill with compost. That'll help, too.

It really depends on the quality and depth of your topsoil, but any type of compost or soil conditioner will never do any harm, if you have clay it will continuously need improving anyway if you are planting it, if you are putting down turf I would suggest adding grit or gravel for drainage.

posted same time spritz!

Snap! At least we agreed.... ;-)))