Buy yourself a paint edge for about $5US. It is a 3 foot long metal edge with a handle that you place along baseboard or moldings of whatever you don't want to get painted.
You paint the wall up to the paint edge. Any paint that gets on the edge you simply wipe off.
You then move the edge to the next location. Paint, wipe, move.
This technique gives a very clean edge with no seepage.
To eliminate splatters on baseboards after rolling, simply wipe them off with a damp cloth to remove them. Use a cloth dampened with mineral spirits for oil paints.