Danaher says:

The second you make contact with the leg – attack your opponents balance: A very common problem you will see in Jiu jitsu athletes is a tendency to simply pick the leg up when going into their high single legs. As a result their opponents can easily maintain balance on one leg and because they are in good balance, they can immediately go into strong counters – Guillotines, Kimura, leg lock entries – all kinds of problems. It is very important that the second you get two hands to the leg, you immediately exert strong downward pressure with shoulder/chest whilst pushing with your head so that your opponent is out of balance with weight in his heels. Often this will force him to use his arms to help regain balance – this precludes him from using this arms to lock up his guillotine, Kimura and leg lock counters. All his attention will be momentarily taken up with staying up rather than attacking you. Then you can focus on the task of completing the takedown without the distraction of defending yourself from his counters. Make a habit doing more than picking the leg up – make sure you go further disturb his balance at the same time and keep him hopping and reaching so they he never gets settled enough to counterattack so you can make a smoother completion to your takedown.