Paris's RATP public transit system operates special small buses called Montmartrobus that run from Place Pigalle over the top of Montmartre (map), stopping near Place du Tertre and the Basilique du Sacré-Coeur, then continuing down the north slope of the hill to the Place Jules Jofferin and the Mairie [City Hall] du XVIIIe arrondissement.

The Montmartrobuses run quite frequently—about every 10 or 12 minutes—and go from the Pigalle Métro station to the very top of the hill at Place du Mont Cenis, in front of the Église de Saint-Pierre de Montmartre, next to Place du Tertre, making this an even better up-the-hill method than the Funiculaire du Montmartre if you don't like to climb steps. (The Funiculaire doesn't start at the bottom of the hill, and it doesn't go all the way to the top.)

Here's a larger version of the bus route map.

Here's a printable version (pdf) of the map.

Of course, you can always climb all those steps if you like. (How are your knees these days?) But it's nice to know you have alternatives....