Imagine you'd like to change the appearance of your form layout depending on values entered in a picklist, for example, you could reveal more fields on a record if a certain value is selected. 

In this example, I've made it so that additional football fields are only available when football is selected as an organisation's favourite sport. 

Default Layout:

Form layout after selecting Football:

Firstly, if it doesn't exist already, you'll need to create a custom field for your picklist - it makes sense to check the Show by Default checkbox for this field so that it appears on all your form layouts by default. Then add the extra fields which will be used to store additional information without the Show by Default box checked, otherwise they'll appear on all your form layouts.

Once complete, you'll need to create a copy of your default form layout for each variation you'll need. I would recommend doing this after you've added the custom field picklist which will drive which form layouts are displayed (in this example, Favourite Sport).

Now with your copy of your form layout, adjust the layout appropriately. You'll need to do this for every variation of form layout you wish to have. Go to our Knowledge Base here for more information on how to do this. 

The key step in making this work is in applying the correct form layout rules.

You will only want the above form layout to be applied when the Favourite Sport field is equal to Football. See below.

And likewise, on your default form layout, you'll only want this to be applied when the field is not equal to any of the other options, like so:

So now, your default form layout will only be selected if one of the options in your picklist is not chosen, otherwise it will display the relevant form layouts. 

NOTE:  If you wish to make changes to your default form layout, you'll also need to make the same changes to all of your form layout variations. Therefore it's not recommended to get to carried away with this approach, otherwise you'll have a lot of form layouts which need to be maintained when making changes.