Disguising Tempo Changes
On rhythmically active music, tempo changes are quite obvious and can drag so much attention on themselves that they might not be appropriate in certain scoring situations. But of course they might be necessary as the scene changes pace etc. A good way to get more smooth tempo transitions is to disguise them.
A very common way is to hold a chord/note for one/two bars without any pulse and then continue in the new tempo. That little gap will smoothen out the transition a bit as it avoids a radical contrast. Disguise is always possible by stopping or making a rhythmical pulse more ambiguous before going to the new tempo.
Another thing that might make it more smooth is to change into a tempo that is related to the old tempo with doubling or halve being the smoothest one. But also plausible fractions like “old dotted quarter length becoming the length of new quarter” will feel quite natural.
It might also be noted that in some cases contrasting tempos are actually a quite clever and well working idea.