Himmler and Bouhler killed themselves after they were in WAllied custody. So they're the easy ones: keep them in handcuffs from the moment of arrest, put them in padded cells, keep them under suicide watch, and make sure the prison guards can be completely trusted not to smuggle lethal objects into the prison.
Bormann is a little more tricky, as he died very close to Lehrter Station (now Berlin Hauptbahnhof). Most likely he killed himself with cyanide, because glass was found in his mouth when his body was exhumed. He came close to the Russians at several points in his circuitous journey to Lehrter Station. If the Russian soldiers had been able to identify Bormann, perhaps they had a chance of taking him alive. They would have had to act quickly, seizing Bormann before he had a chance to bite on his cyanide capsule.
Goebbels and Hitler are the hard ones, with Hitler being harder than Goebbels. I can't see it happening with the OTL military situation in Berlin, or with the OTL personalities of Goebbels and Hitler. (That is, no ASBs convincing Hitler that suicide is wrong.) Placing them in "protective custody" on a sort of suicide watch might have kept them alive, but who could possibly have given an order to arrest the Führer? Alternatively, have Hitler and Goebbels go to Berchesgaden instead of Berlin. Then have the SAS or some other special forces parachute into the area, and have them storm the Berghof at 4 in the morning. Given a quick, decisive assault and a lot of luck, Hitler and Goebbels could have been caught as they slept. It's a stretch, but it could be done without ASBs.