As some have suggested, pre-stretching would not help. It's not about tension loss, it's about what I would call "elasticity" which is gone after 5 hours or so.
1. For the past 10 years or so, the prevailing wisdom around the boards has been exactly what is stated above ... that poly loses elasticity with use, and this is the cause of string "death". However, the March 2013 study by the TW Professor indicates that this is not the case. We can say that poly does not lose elasticity with certainty, because poly string that that has been hammered repeatedly until it suffers significant tension loss, behaves in a similar fashion to fresh poly, provided it is simply re-tensioned to the same starting point. During testing, the string was actually hammered on until the tension dropped to
zero, which far exceeds any stress the average player will subject their string bed to. Poly death is simply a matter of two factors, tension loss, and increasing friction due to notching.
How Tennis Strings "Go Dead" — Part 1
The Change in String Properties with Repeated Impacts
http://twu.tennis-warehouse.com/learning_center/deadstrings.php
How Tennis Strings "Go Dead" — Part 2
Do Strings Lose Elasticity with Repeated Impacts?
http://twu.tennis-warehouse.com/learning_center/deadstringsPart2.php
2. Black Widow is actually above average in terms of tension maintenance
for poly (which is lousy overall compared to gut or nylon) The 17 gauge version ranks as 15th best, (Ref tension 51, medium swing) out of 256 polys tested with Lux 4G holding the top spot.
3. Figure that Poly in general will lose about 20% tension overall. Quite a bit of that 20% will occur within the first 24 hours after stringing. Below is a quote from Paul / UK Skippy concerning tension loss. Paul has been on the Wimbledon String Team since 2004.
Strings start to loss tension immediately after stringing. Tension lost will depend on what string you've used.
Poly will lose about 20%, Syn guts about 10%, and natural gut 5%. They will also settle after the initial loss. If you're not planning to use the rqt within a few days of it being strung, then get it strung nearer the time to you using it. - Regards, Paul
http://tt.tennis-warehouse.com/showthread.php?p=7258204#post7258204
-Jack