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;
	}
});

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,'');
}