SELECT CustomerNumber,

SUM(CASE WHEN [YearMonth] BETWEEN 201401 AND 201412 THEN TotalAmount ELSE 0 END) AS Sales_2014,

SUM(CASE WHEN [YearMonth] BETWEEN 201201 AND 201312 THEN TotalAmount ELSE 0 END) AS Sales_2012_2013,

SUM(CASE WHEN [YearMonth] <= 201112 THEN TotalAmount ELSE 0 END) AS Sales_Prior_To_2012

FROM dbo.tblYearMonthTotals

GROUP BY CustomerNumber

HAVING

SUM(CASE WHEN [YearMonth] BETWEEN 201401 AND 201412 THEN TotalAmount ELSE 0 END) > 0 AND

SUM(CASE WHEN [YearMonth] BETWEEN 201201 AND 201312 THEN TotalAmount ELSE 0 END) = 0 AND

SUM(CASE WHEN [YearMonth] <= 201112 THEN TotalAmount ELSE 0 END) > 0

Edit: Frankly in this kind of situation, I think the YYYYMM actually makes it easier to code than a date/datetime, but maybe that's just me.

Edit2: I also assumed the possibility of returns, for example, +$125.00 in one month, then -$125.00 in the next month, resulting in a net of $0.