To keep the House & Senate Democratic, you're likely going to have to make Bill Clinton less aggressive in pushing his agenda in those first two years. So, no healthcare push and no assault weapons ban, which galvanized the right and fired up the conservative base.
But like with 2010, I'm not sure there's enough momentum to change the dynamics. Very similarly, Democrats won huge majorities due to their success in Republican-leaning districts. Those are generally harder districts to keep, especially when there's a backlash to the party in power.
The economy was also an issue in 1994. While it was improving, it still hadn't hit that 90s boom and it played a role. All that together killed Democrats' chances. They were likely going to lose seats in that election no matter what but maybe there's enough to hold on to those seats without the failed healthcare push and the assault weapons ban.
Or you could go the more radical route and have a terrorist attack in mid-94 that shifts the narrative and the Democrats sweep to victory in a rally around the flag narrative.