Two things Celery love is manure and water, so dig in plenty of well rotten manure.
If you have any old 6" plant pots cut the bottoms out and place over the plants you do this only to support the outer stems, remove any side shoots as they grow.
About mid July onwards when the plant is at its thickest collar them with cardboard two thirds up the plant, you can raise the collar as it gets taller.
Keep the plant moist but not wet to avoid heart rot.
Celery is very attractive to slugs so if you dont mind using slug pellets then use them.