It all depends on what's in your tank
If you want them to feel safe and come out you need a lot of plant cover (especialy floating) always near by. The plants need not be real and usualy they need not even be plants - just a lot of cover.
The other thing that helps is to not have any large, boisterous or very active species in the same tank. Pearls usualy shy away from other gouramies - especialy larger ones - or larger fish.
Active shoaling fish can also be a problem as they may intimidate the pearls. Peaceful shoaling species help encourage pearls to show themselves as they are a sign of a secure environment. Fish like harlequin rasboras or peaceful tetras would fall into this category.
In a large tank, bottom-dwellers don't realy matter provided they don't come near the nervous pearls at the surface.
Usualy, even the most nervous and shy fish will get used to their new environment and eventualy grow to be a lot more outgoing and confident. Changing the inhabitants and adding new, unfamiliar fish species, will cause the pearls to be even more nervous.
For food, try feeding all the other fish on one end and the pearls on the other. Tempt them with live or frozen foods.