Its almost impossible to, but I'd suggest having more armed police at crowded events such as marches, and have more active security patrolling streets. Also crack down on black markets for selling firearms, while also encouraging people to watch out for signs of radicalisation and to report it. And if someone is on a terrorist watchlist of any kind, they should have their property searched and monitored closely, instead of following Europe's past strategy of knowing they're there but ignoring the threat.