By Chris O'Regan, Amateur Titanic enthusiast from Brisbane
1. Titanic could have been constructed with a double hull. The technology to construct double hulls was available; SS Great Eastern had been launced with a complete double hull over 50 years earlier, in 1858. In a classic failure of risk management, Titanic's manufacturers, alongside with most shipbuilders at the time, considered a full double hull an unnecessary expense, being satisfied with a double bottom instead. This all changed after the disaster, with liners everywhere being refitted with full double hulls. Suddenly the expense didn't seem to matter so much.
2. The quality of the riveting and steel plates could have been better. In the present day, ship plates are welded together using oxyacetelene torches. This technology was unvailable in Titanic's time. Instead, Titanic's overlapping steel hull plates were held together by rivets that were hammered in by hand. Most of the rivets were steel, but some were made of wrought iron; according to some, the rivets were poor-quality and contained large amounts of slag. The actual holing of the ship was caused when she dragged over the surface of the iceberg, with the berg snapping or popping the rivets along the hull, allowing water to enter in between the hull plates. The hull plates themselves are alleged to have not been strong enough, with signs of stress fracturing; but this is disputed.
3. The ship's watertight bulkheads could have been extended and fully sealed to reduce the risk of flooding. Titanic was constructed with transverse bulkheads (i.e. walls) to divide the ship into 16 watertight compartments, which could be sealed off with doors operated either manually or remotely from the bridge. So far, so good. However, the bulkheads didn't extend up the height of all decks, and weren't sealed at the top. So while flooding could be safely be contained if only a small number of compartments were flooded, if too many flooded, water would reach over the top of the bulkheads and flood the rest of the compartments until the ship sank. The designer of the ship, Thomas Andrews, was on board and consulted by Captain Smith immediately after the iceberg was hit. He quickly realised that with five compartments flooded thanks to the iceberg dragging along the ship, the flooding could not be contained and the ship would eventually sink. His calculations convinced Smith to begin evacuations and therefore probably saved lives. Bulkhead design on subsequent ships would be improved as a result of the disaster.
4. Captain Smith could have responded to the numerous ice warnings the ship received by slowing down or stopping completely and waiting for daylight. Titanic was radioed several times by several different ships to warn of large amounts of ice in the area. Titanic acknowledged these warnings, but continued to cruise ahead at full speed. Captain Smith did respond to the warnings; he changed the ship's course to be more southerly, and he posted lookouts to watch specifically for icebergs. One of those lookouts did in fact spot the iceberg, but not with enough time to avoid a collision, since it was a moonless night and Titanic was travelling at around 20 knots, close to her maximum speed. This might seem insane to us now, but passenger liners had a very strict schedule to keep, and ships of her size and construction weren't considered to be vulnerable to icebergs (the 1997 movie implied Titanic was trying to break a speed record at the behest of J. Bruce Ismay, but that is fictional).
5. The wireless (i.e. radio) operators could have passed on the ice warnings with more urgency. One incident in particular became notorious. The nearest ship to the Titanic before she sank was SS Californian; her captain had decided the ice was so bad that he would stop and try to resume the journey at first light. Californian's wireless operator signalled to Titanic about this. Unfortunately, that message came right at the time that Senior Wireless Operator Jack Phillips was attempting to get through a backlog of passenger messages he hadn't been able to send off earlier (because the set had been broken earlier and Titanic hadn't been in range of the nearest wireless station, Cape Race, Newfoundland). Californian's signal broke in over the top of Phillip's broadcast (these were wireless spark-gap transmitters that used morse code; signals couldn't be tuned out) and was very loud in his headphones because the ships were so close. Phillips angrily replied "Shut up! Shut up! I'm working Cape Race!" Although this was not the only warning Titanic received, it happened less than ten minutes before the collision, so it might possibly have made a difference if Phillips had been paying more attention and had relayed it promptly to the bridge. A warning before that, from SS Mesaba, had not gone up to the bridge because Phillips was busy processing the passenger messages.
6. Finally, once the iceberg had been spotted, the officer on watch, First Officer William Murdoch, could have reacted differently in trying to avoid a collision. Some have suggested that Titanic should have not attempted a course change and simply steamed over the top of the iceberg; I highly doubt this would have helped. But one thing that might have helped is if he hadn't ordered "Full Astern (i.e. reverse)" as he attempted to steer around the berg. Ships of the period did not have a throttle accessible on the bridge; instead, orders to change speed or direction were relayed to the engine room by a device known as an engine room telegraph (the famous circular brass contraption). Once the message was received at the engine room the engineers had to spend a few moments getting the ship's enormous engines to respond and switch to reverse (the steering gear, as well, took time to respond as the steam-powered rudder moved into position). If the Titanic had not been slowing down as she approached the iceberg but instead continuing at full speed, she might have been more manoeuverable, able to turn harder and avoid the iceberg entirely. But this is speculation; there was only a very short amount of time (around 40 seconds) for Murdoch and the engine room to react, and something else could easily have gone wrong.
There are plenty of other things that could have been done to make the sinking less calamitous when it did happen (for example, the ship needed more lifeboats), but these are the main things that could have made a difference prior to the collision itself. Once the hull had been breached across more than four compartments, which happened immediately after the iceberg struck, the ship was doomed.