When you first agree the deal with your buyers, agree a specific date by which exchange of contracts should take place. This doesn’t have any legal effect, but it gives you a justification for exerting pressure as the day approaches and there is no sign of the buyers being ready to exchange.
You can announce, for example, a week ahead that if exchange of contracts does not occur on the due date then you are putting the property back on the market or even withdrawing (if you have the nerve). If the delay is due to the buyers’ solicitor being slow or the buyers not bothering to get their act together, this can have a galvanising effect. The solicitors certainly won’t want to be blamed for the deal falling through.
A slightly different strategy – the one I prefer, in fact, because it is stronger – is that you should tell the buyers at the start that you will take the property off the market now that the deal has been agreed, but you will put it back on again if they do not exchange by a particular date. That will concentrate the buyers’ and their solicitors’ minds on meeting the target date right from the start.