Scotland votes leave and he is made Prime Minister of the transitional Goverment, before handing over to either Sturgeon or one of his other acolytes.
OR
Debbie Horton never tells him to "If you feel like that, go and join the bloody SNP" (which is generally credited as what triggered him joining the SNP) whilst he's at St. Andrews. Instead, Salmond ends up joining Labour, and whilst doubtful they'll give devolution to Scotland, is encouraged by the Scotland Act of '78 to stay in Labour. He doesn't become an MP immediately, and instead spends several years in a banking position, being picked up as a Parliamentary candidate in '83 or '87, or a by-election between them, becoming a vocal proponent of Scottish Devolution. He's eventually put into the Shadow Chief Secretary to the Treasury after Brown is elevated to the Shadow Chancellery, with Blair eventually appointing him Shadow Secretary of Scotland (assuming Salmond doesn't disrupt the butterflys too much). Labour wins, and Brown ultimetly leaves office following the loss of his daughter (assuming minimal butterflys and he still looses his daughter) and the breakdown of him and Blair's relationship, with Salmond becoming tapped to become Blair's successor. Blair eventually decides to step down, Salmond facing off against Brown in a Leadership contest, Salmond prevailing and declared the victor, becoming Prime Minister.