Here is what we use ( in a static class )
Be aware that Cache is global and NOT session based.

       #region checkCache
       /// <summary>
       /// Will test for the requested value in the Session Cache.
       /// </summary>
       /// <typeparam name="T">Type of cached object</typeparam>
       /// <param name="sKey">Key to search on</param>
       /// <returns>T</returns>
       public static T checkCache<T>(string sKey)
       {
           T returnValue = default(T);

           if (HttpContext.Current != null)
           {
               object oValue = HttpContext.Current.Cache.Get(sKey);

               if (oValue is T)
               {
                   returnValue = (T)oValue;
               }
           }

           return returnValue;
       }
       #endregion

       #region cacheItem
       /// <summary>
       /// Cache items to the session chache for a set number of
minutes.
       /// </summary>
       /// <param name="sKey"></param>
       /// <param name="oItem"></param>
       /// <param name="cacheMinutes"></param>
       public static void cacheItem(string sKey, object oItem, int
cacheMinutes)
       {
           if (HttpContext.Current != null && oItem != null)
           {
               Cache cache = HttpContext.Current.Cache;

               object oExistingItem = cache.Get(sKey);

               // If the item is already cached
               if (oExistingItem != null)
               {
                   cache.Remove(sKey);
               }

               cache.Add(sKey, oItem, null,
DateTime.Now.AddMinutes(cacheMinutes), Cache.NoSlidingExpiration,
CacheItemPriority.Normal, null);

           }
       }
       #endregion

       #region expireCacheItem
       /// <summary>
       /// Removes an item from the Cache.
       /// </summary>
       /// <param name="sKey"></param>
       public static void expireCacheItem(string sKey)
       {
           if (HttpContext.Current != null)
           {
               HttpContext.Current.Cache.Remove(sKey);
           }
       }
       #endregion

An example of calling that code using the Session ID to make is
session specific in this case.

               string sCacheKey = "CMSContext{0}{1}";
               sCacheKey = string.Format(sCacheKey,
loggedOnUser.GUID, Session.SessionID);

               CMSContext currentContext =
util.checkCache<CMSContext>(sCacheKey);

               if (currentContext == null)
               {
                   currentContext = new CMSContext(Session,
loggedOnUser);
               }

               // Unlike most cache Items we refresh the Cache of
this Item every time it is accessed
               util.cacheItem(sCacheKey, currentContext, 20);