I usually train them to go where I want them to go round the trellises even if that means horizontal. I doubt if nipping out the top would make them flower better elsewhere on the plant so can't really see the point.

I do that too Sarraceniac, I have about 30 clematis, every few days I go out and trail them where I want them to go otherwise my flowers are on the top of the garage. On one of my my clematis this year, instead of trailing it hard on the tressis I put loads of tall sticks around it and this one seems to have got lovely and bushy itself just winding itself around the twigs and clinging to other stems, so next year I may try this method on all of them.

Which varieties of clematis do you have Sunbeam?

As Sarraceniac suggests, its a pointless exercise - nip out the tops and you lose the flowers. Put up a taller trellis, or train it as it grows as suggested, or buy shorter clematis. Some reach 16 feet, others only 8 feet. Not counting the species such as Montana, obviously, which can get 35 feet.

Thank you all, one of them is pink fantasy, the other polish sprit

Okay C. pink fantasy grows to over 10 foot and C. polish spirit to around 15 foot so you will need to train them to grow sideways. They are both late flowering clematis and should be pruned after flowering in autumn.