This technet page shows the methods available for the different types of controls.

Here is an example of how to change the optionset based on a yes/no field:

function BuildOptionSet(picklist, valuelist)
{
	picklist.clearOptions();
	for(var i=0; i<valuelist.length; i++)
	{
		var listitem = valuelist[i];
		picklist.addOption(listitem[0], listitem[1]);
	}
}

var lista = new Array();
lista[0] = new Array('Alpha',0);
lista[1] = new Array('Beta',1);

var listb = new Array();
listb[0] = new Array('Charlie',2);
listb[1] = new Array('Delta',3);

var uselista = Xrm.Page.data.entity.attributes.get("YesNo").getValue();
var optionset = Xrm.Page.ui.controls.get("OptionSet");

if (uselista==true) {
   BuildOptionSet(optionset, lista);
} else {
   BuildOptionSet(optionset, listb);
}

You just need to change the values of lista and listb and the YesNo and OptionSet lines to reflect the fields on your form, and add it to the onchange event of the dropdown Yes/No form field.

setVisibility('section','new_mypicklist','section_main',42);

This hides the section_main section if the value is not 42, which simplifies the process of hiding and showing relevant information.

I also like to use checkboxes to handle the visibility of elements, but checkbox values dont’ change until they lose focus. To work around the issue, I register an onClick event during the form load instead of using the onChange event. This unsupported method allows me to modify the value onClick and change the visibility immediately. To use this I just need to call it in the onload:

registerToggle('section', 'new_checkbox', 'section_main');

If you wanted to hide it when new_checkbox was false (or no), then you would need to specify the value, 0.

registerToggle('section', 'new_checkbox', 'section_main',0);

JScript: visibility.js

This is the main javascript file… visibility.js that you would need to add and reference as a web resource in CRM 2011, everything except for the registerToggle uses supported methods.

function registerToggle(t, attr, c, v) {
    if (typeof (v) === "undefined" || v === null) {
        var v = 1;
    }
    var ctrl = Xrm.Page.ui.controls.get(attr);
    var a = ctrl.getAttribute();
    var el = document.getElementById(attr);

    // Build Toggle Function
    var f = "var ef=function() { " +
              "var a = Xrm.Page.data.entity.attributes.get(attr); " +
              "a.setValue(!a.getValue()); " +
              "setVisibility('" + t + "','" + attr + "','" + c + "'," + v + ");" +
              " };";

    eval(f);

    // Attach to click event
    el.attachEvent('onclick', ef, false);

    // Set visibility
    setVisibility(t, attr, c, v);
}

function setVisibility(t, attr, c, v) {
    switch (t.toLowerCase().charAt(0)) {
        //tab
        case 't': setTabVisibility(attr, c, v);
            break;
        //section
        case 's': setSectionVisibility(attr, c, v);
            break;
        //control
        case 'c': setControlVisibility(attr, c, v);
            break;
        //navigation
        case 'n': setNavigationVisibility(attr, c, v);
            break;
    }
}

function setNavigationVisibility(attributename, navitemname, value) {
    var attribute = Xrm.Page.data.entity.attributes.get(attributename);
    var navitem = Xrm.Page.ui.navigation.items.get(navitemname);
    if (navitem === null)
    {
        return;
    }
    navitem.setVisible(attribute.getValue() == value);
}

function setTabVisibility(attributename, tabname, value) {
    var attribute = Xrm.Page.data.entity.attributes.get(attributename);
    var tab = Xrm.Page.ui.tabs.get(tabname);
    if (tab === null)
    {
        return;
    }
    tab.setVisible(attribute.getValue() == value);
}

function setSectionVisibility(attributename, sectionname, value) {
    var attribute = Xrm.Page.data.entity.attributes.get(attributename);
    var tabs = Xrm.Page.ui.tabs.get();
    for(var i in tabs) {
        var tab = tabs[i];
        var section = tab.sections.get(sectionname);
        if (section !== null) {
            section.setVisible(attribute.getValue() == value);
            return;
        }
    }
}

function setControlVisibility(attributename, controlname, value) {
    var attribute = Xrm.Page.data.entity.attributes.get(attributename);
    var control = Xrm.Page.ui.controls.get(controlname);
    if (control === null)
    {
        return;
    }
    control.setVisible(attribute.getValue() == value);
}

Code reuse is important, and I believe in creating libraries and using libraries. Javascript files are libraries, and they are cached. So if you update a Javascript file you are loading this way, make sure your users clear their cache.

function loadjs(filename, onload){
  var scriptfile=document.createElement('script');
  scriptfile.setAttribute("type","text/javascript");
  scriptfile.setAttribute("src", filename);
  scriptfile.onreadystatechange=onload;
  document.getElementsByTagName("head")[0].appendChild(scriptfile);
}

loadjs("/MoreJavascript/tools.js", function(){
	if (this.readyState == 'loaded' || this.readyState == 'complete') {
		FunctionInJavascriptFile();
		this.onreadystatechanged=null;
	}
});

CRM 2011 uses web-resources which can be a common JScript file.

Recently on the Microsoft forums someone asked how to change the values of a picklist based on the yes no value of a boolean field. Just add this script to the OnChange event of the yes/no field.

This technet page shows some of the functions on different form fields.

Here is an example of how to change the picklist based on a yes/no field:

function CreatePicklistValue(Name,Value)
{
	var retval = new Array();
	retval[0]  = Name;
	retval[1]  = Value;
	return retval;
}

function BuildPickList(picklist, valuelist)
{
	var options = picklist.Options;
	for(var i=0; i<options.length; i++)
	{
		var option = options[i];
		picklist.DeleteOption(option.DataValue);
	}
	for(var j=0; j<valuelist.length; j++)
	{
		var listitem = valuelist[j];
		picklist.AddOption(listitem[0], listitem[1]);
	}
}

var lista = new Array();
lista[0] = CreatePicklistValue('Alpha',0);
lista[1] = CreatePicklistValue('Beta',1);

var listb = new Array();
listb[0] = CreatePicklistValue('Charlie',2);
listb[1] = CreatePicklistValue('Delta',3);

var uselista = crmForm.all.YesNo.DataValue;
var picklist = crmForm.all.Picklist;

if (uselista==true) {
   BuildPickList(picklist, lista);
} else {
   BuildPickList(picklist, listb);
}

You just need to change the values of lista and listb, and the crmForm.all.YesNo and crmForm.all.Picklist.

if (crmForm.all.new_name.DataValue && crmForm.all.new_name.DataValue.length >0)
{
crmForm.all.new_name.DataValue = crmForm.all.new_name.DataValue.replace(/[^a-zA-Z 0-9]+/g,'');
}