In my company we share the PF / PNF roles.
It is usually decided in the crew room based on the weather / notams / general preference.
A FO has a X wind limit of 15kts, a SFO 25kts.
If the RVR for take off is < 400m, or LVP for landing, or windshear reported, or contaminated runways, or runway width < 45m, or a technical problem affecting handling the Captain will be pilot flying.
Some airports also have restrictions such as a difficult circling procedure may be captains only.
Other than that, it is split quite fairly and on a typical 4 sector day you would be PF for 2 of the flights.
The FO will be starting the engines / taxiing on his sector and essentially the only bits he won't be doing is guarding the thrust levers on take off (skippers decision) and parking the plane on stand.
Hope that gives an insight.
Last edited by EcamSurprise; 24th Mar 2013 at 14:46.