When I do things like this I usually use some kind of reverb to fade out the first track and bring in the new track. If you dont overuse it I think it sounds pretty rounded up, especcially when the second track has some catchy instrumental or voical going on that may even is somehow connected with the first track.
When you are using Serato I can with no doubt recommend the “Epic Reverb” effect for this which can be found in the free FX-pack (I still dont understand why they dont pack it in by default).
yeah, i remember reading a bunch of articles and posts on here somehwhere talking about your exact question with TONS of other DJs giving their opinions below the article.
Off the top of my head, number of ways, i think using FX is one, acapellas is another, using different versions of the same song to create a unique mix of the two to bring it either lower or higher.
The stop-n-drop is always a good one for big changes like that… on the 3rd beat of the last bar you want to play, press stop on the outgoing deck, wait one beat, then press play on the incoming. It can work nicely with a record-stop effect as part of the stop.