You can use train only, first direct train to Wrocław, then to Lwów directly or maybe to Kraków, then Lwów. From Lwów you catch a Ukrainian train or bus to Lutsk and return to Prague, through Lublin-Wrocław-Prague route.

I you want to visit Lutsk and Lviv you should go to Lublin. You can go to Wrocław, then take the train to Lublin, then go to the bus to Lviv/Lutsk. You can also go there through Warsaw, from Prague directly.

Second way is to go through Cieszyn - Katowice/Kraków, then to Lviv directly.

I did not use Polskibus, but it's definitely better than small "busy" which are popular in Eastern Poland. You should be prepared for worse quality of Ukrainian buses which are used on the route from Lublin to Lutsk/Lviv.

If you use train : www.rozklad-pkp.pl/
If you are looking bus in the city ( e.g. Wroclaw, Warszawa) : www.jakdojade.pl
Cheap flight from Wrocław to Warszawa : http://www.ryanair.com/pl/ .. sometimes flight to Warszawa cost 19 PLN.
As wrote Aneta basically polskibus.com have connection your destination. It's depends what are you what to visit by the way.

Polskibus.com if you're looking for bus connections from Prague. Prices are really nice and the faster you book a ticket, the smaller are all prices :)