Optimization of Stored Procedure

I'm trying to optimize a stored procedure I have which will insert new records into a growing table.  Currently the table has about 9 million rows in it, and would grow by ~20,000 a day in the first insert.

The 2nd insert will not have too many records being modified, I'd say about 500 a day.

Currently my table has a clustered index on ESN (can't put unique as this table will have multiple records per ESN), and a regular index on DateAsOf.

When I look at the Query Execution Plan, my highest percentages are on Sorts.  Is there anything better I can do to speed this up?

Thanks..

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create procedure proc_update_tbl_esn_promocode_History as
insert into tbl_esn_promocode_History
select esn, promo_code, convert(datetime, convert(varchar(11), getdate()-1)) as DateAsOf,
'Y' as IsCurrent
from esn1 where esn not in (select esn from tbl_esn_promocode_History)

insert into tbl_esn_promocode_History
select e.esn, e.promo_code, convert(datetime, convert(varchar(11), getdate()-1)) as DateAsOf,
'Y' as IsCurrent
from esn1 e, tbl_esn_promocode_History t
where e.esn = t.esn
and t.iscurrent = 'Y'
and ((t.promo_code <> e.promo_code)
or (t.promo_code is null and e.promo_code is not null)
or (t.promo_code is not null and e.promo_code is null))

update tbl_esn_promocode_History
set IsCurrent = 'N'

update tbl_esn_promocode_History
set IsCurrent = 'Y'
from tbl_esn_promocode_History t, (select esn, max(DateAsOf) DateAsOf
from tbl_esn_promocode_History t
group by esn) t1
where t.esn = t1.esn
and t.dateasof = t1.dateasof

GO