You need to implement a type converter for your property.
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlr
fsystemcomponentmodeltypeconverterclasstopic.asp

For example, if you have a string property that you want to limit to a few
choices, create a class like this:
public class MyConverter : StringConverter
{
 public override bool GetStandardValuesSupported(ITypeDescriptorContext
context)
 {
  //true means show a combobox
  return true;
 }
 public override bool GetStandardValuesExclusive(ITypeDescriptorContext
context)
 {
  //true will limit to list.  false will show the list, but allow free-form
entry
  return true;
 }

 public override
System.ComponentModel.TypeConverter.StandardValuesCollection
GetStandardValues(ITypeDescriptorContext context)
 {
  return new StandardValuesCollection(
   new string[] { "entry1", "entry2", "entry3" });
 }

}

Then hook it up to your property like this: (note the [TypeConverter]
attribute)
private string _myProp = "entry1";
 [Browsable(true)]
 [DefaultValue("entry1")]
 [CategoryAttribute("Behavior")]
 [TypeConverter(typeof(MyConverter))]
 public string MyProp
 {
  get{ return _myprop;}
  set{ _myprop = value;}
 }