Here's how to access each control on your forms using Javascript. Using this information you can add very useful functionality to CRM's front end.
There are a couple of ways to do this, you can choose which ever suits your situation best...
Currently displayed entitie's GUID:
crmForm.ObjectId
To access a control on the form:
crmForm.all.[attributename]
// OR
document.getElementById("[attributename]")
Hide/show attribute(set to visible or invisible):
//Hide it
crmForm.all.[attributename].style.display = "none";
//Show it
crmForm.all.[attributename].style.display = "inline";
Enable/disable attribute:
//Disable
crmForm.all.[attributename].Disabled = true;
//Enable
crmForm.all.[attributename].Disabled = false;
Does control/attribute contain any data?
if (crmForm.all.[attributename] != null)
//Contains some data
Control's value(this is the current on screen value. ie, may not have been saved yet):
crmForm.all.[attributename].DataValue
// OR
crmForm.all.[attributename].value
// OR if its a checkbox
crmForm.all.[attributename].Checked = [true false=""]
// OR if its a lookup
//Descriptive name for the selected item
crmForm.all.[attributename].DataValue.name
//GUID value of the selected item
crmForm.all.[attributename].DataValue.id
//Type of entity selected in the lookup(1=Account, 2=Contact...)
crmForm.all.[attributename].DataValue.type
//Descriptive name for the selected entity type
crmForm.all.[attributename].DataValue.typename