Paris to Poitiers by train

Note that these trains to Poitiers depart from the Gare Montparnasse in Paris.

Final Destination: Bordeaux or La Rochelle or Poitiers

The departures with a journey time of 1hr 18mins (around 50% of all departures) are non-stop between Paris and Poitiers and they use the new high speed line; the LGV Bretagne-Pays de la Loire.
These fastest trains are not automatically more expensive.

The departures with a journey time of around 1hr 35mins still take the older route via St Pierre des Corps.
Poitiers is comparatively close to where the extension to the high speed line (the LGV Sud-Ouest) begins, hence the newly routed non-stop trains not being particularly faster.

Though if possible avoid the departures with a journey time of more than two hours, they're slower because they call at more stations and on departure from Paris they are joined to another train heading to Tours.
It's the separation of the two trains at St Pierre-des-Corps station which extends the journey time.

Book early and save: Yes

Online bookings usually open: up to 6 months ahead of the travel date - but bookings can open sooner when SNCF is running a promotion.

When booking 2nd class tickets only one type of ticket will be available per departure, discounted 'Prems' tickets or 'Seconde' tickets; though the price of Seconde tickets are also typically cheaper when they are first placed on sale.
When booking 1st class tickets there will be a choice of Premiere tickets and Business Premiere tickets.

The two key T&Cs of booking Prems, Seconde, or Premiere tickets are:

1. Up until two days prior to departure, these types of tickets can be refunded for no additional charge (big tick), but a €15 fee will be payable for making a refund in the two days prior to departure.
2. It's also possible to exchange tickets to an alternative departure on the same journey, but you will have to pay any difference in price between the cost you paid and the cost at the time of the exchange.

Though if you book Business Premiere tickets, the key T&Cs are that tickets can be exchanged and refunded free of charge up to 30 minutes after departure, so if you get to the station just too late to miss the train you were originally booked on to, you won't lose out financially.

Seats will automatically be assigned when booking tickets for this journey, but rail pass users will need to be pay a reservation fee prior to boarding these TGV trains.

The price is dictated by demand per departure, so book ahead and try and be flexible re: your departure and arrival times, as trains leaving earlier or later can be cheaper.
SNCF Connect will indicate the cheapest prices per day.