Pickling whole cucumbers
We (my family) never had a problem, cucumbers always turned out crispy. I don't have exact recipe, it's all "when it tastes just right", but here are general steps:


Select small cucumbers. 2 - 5cm, 1 - 2 inch long. 
Boil seasonings, water (and sugar if you want it) together. About 15 minutes of boil should be enough.
When you turn off the heat, add 1 cup of 10% vinegar for every 4 cups of water. May be bit more, may be bit less, it's taste preference.
Put washed cucumbers in jars. Tight but without forcing them. Add fennel (umbels), garlic, mustard seeds.
When marinade is cool, pour it to jars, it should cover your cucumbers. It does not need to be cold. If you can put your finger in it and don't feel uncomfortable, it's OK. If in doubt, assume it's too hot.
Put caps on jars and pasteurize them. 10 minutes of boil is abuot enough, but go by colour. Cucumbers will start to go yellowish. When almost all surface changed colour, it's time to pull jars off and left them to cool.
We let them cool upside down.


That's about it. Don't know which step is essential for crispness. For me and mine, this is what always worked.