Regular metal film and carbon film resistors are meant to operate at a relatively high surface temperature, so the best way to run them at full rating is to mount them at least the body diameter off the board on the leads. There were lead formers that did this with a single tool, but a good compromise is some small needle nose pliers to put a kink in the leads at the right point so they are spaced there for soldering. As Tim said, a large copper pour on the solder leads will keep those cool, and help prevent solder joint failure, but also have a large solder mask relief as well, or leave the whole pad free and solder fill the pad entirely.
Power resistors ( typically a white ceramic boat with the element inside and filled with a cement, or a green cement coat on the resistor wound on a ceramic or glass fibre former with pressed on end caps) are best mounted vertically, with use of a special tinned steel mounting that provides one lead connection, along with holding the resistor from moving with vibration, that spaces it typically 1cm from the board. Otherwise mount them 3cm from the board, using either small ceramic beads or a ceramic or metal tube, though there are solder in rivets that combine the tube with support as well. Power resistors typically are rated for full dissipation at a surface temperature of something between 175C to 200C, and really should be derated to reduce the high surface temperature so you do not scorch the board, or run them mounted to tag strips so that the heat is off the board, and use wires to connect them.
Worst thing you can do is have power resistors mounted right next to electrolytic capacitors, where you cook the capacitors to failure in short order. Even worse id to have them touching, or use silastic or glue to hold them to the board or other components, which will cook the board brown and make tracks peel off when you try to rework there