T-SQL (SS2K8) :: Add Balance Of Previous Row To Current Row

I am novice to intermediate writer of T-SQL. Here is my current Query:

SELECT [FISCALYEAR],
[ACCTPERIOD],
SUM([ACTIVITYDEBIT]) AS TrialBalanceDebit,
[POSTINGTYPE]
FROM [dbo].[TB_Lookup]
WHERE [POSTINGTYPE]='Profit & Loss'
GROUP BY [FISCALYEAR],[ACCTPERIOD], [POSTINGTYPE]
ORDER BY acctperiod ASCand this is what is produces.

FISCALYEARACCTPERIODTrialBalanceDebitPOSTINGTYPE
2014 201401 282361372.13000 Profit & Loss
2014 201402 227246272.86000 Profit & Loss
2014 201403 315489534.33000 Profit & Loss
2014 201404 287423793.76150 Profit & Loss
2014 201405 256521290.76000 Profit & Loss
2014 201406 65582951.30000 Profit & Loss

Now I need a way to add another field that takes the TrialBalanceDebit from current ACCTPERIOD and adds it to the Previous ACCTPERIOD TrialBalanceDebit.