Poland:
Prevent the split in the Freedom Union that led to the creation of PO. This can be done in several ways - you could have Geremek prevent his allies from purging Tusk's faction at the 2000 congress, or better yet, have them actually run a presidential candidate in the 2000 election so Olechowski doesn't do so well in the first place and he doesn't get the popularity boost that allowed the creation of PO in the first place. In any case, if UW actually manages to enter the Sejm in 2001, you will probably see an SLD-UW coalition because that's the way Geremek was already inclined (especially given his centre-left posturing during the campaign), Miller probably won't mind considering how well it'd align with his newly-developed neoliberal instincts, and Kwaśniewski will push for it hard. Meanwhile, PiS or LPR, with their welfarist and protectionist leanings, become the main force on the right. This means that there is no explicitly economically liberal party on the right. (Yes, believe it or not, PO was seen as a right-wing party once)
An SLD-UW coalition effectively butterflies the Rywin affair - that's not to say it definitely won't happen, but if it still does Michnik isn't going to reveal an affair that could destroy the coalition of his dreams. The SLD will still have plenty of corruption scandals, but it will avoid having all of its dirty laundry aired out on live TV for a full year which is what tarnished them so comprehensively IOTL. This means that the coalition is probably re-elected in 2005, if only because the alternatives aren't seen as credible enough to win yet, which will vindicate Miller's 'hard neoliberalism, avoid cultural liberalism at all costs' approach. This gets you a party system with a neoliberal, moderately socially conservative (in practice if not always in rhetoric) party on the left and a strongly socially conservative and economically left-wing (in rhetoric if not always in practice) party on the right.
I don't think I can do any better than that.