use of People Picker programmatically in web-parts and pages

Add
<%@ Register TagPrefix="Utilities" Namespace="Microsoft.SharePoint.Utilities" Assembly="Microsoft.SharePoint, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Import Namespace="Microsoft.SharePoint" %>
in the page.


 <SharePoint:PeopleEditor ID="spPeoplePickerUserName" Width="200" runat="server" SelectionSet="User"     AllowEmpty="true" MultiSelect="false" AutoPostBack="false" />




to store the data use

if (spPeoplePickerRespUser.ResolvedEntities.Count > 0)
                                        {
                                            PickerEntity selectedEntity = (PickerEntity)spPeoplePickerRespUser.ResolvedEntities[0];
SPUser user = oWeb.EnsureUser(selectedEntity.Key);
                                itemToAdd["Entered_x0020_By"] = user;
                                        }



for edit mode,

spPeoplePickerUserName.CommaSeparatedAccounts = Convert.ToString(itemToUpdate["User_x0020_Name"]);


 if (spPeoplePickerUserName.ResolvedEntities.Count > 0)
                        {
                            PickerEntity entity = (PickerEntity)spPeoplePickerUserName.ResolvedEntities[0];
                            accountName = entity.Key;
                            int pos = accountName.IndexOf('\\');
                            accountName = accountName.Substring(pos + 1);
                            SetContactInformation(accountName);

                        }