How do you decide what the sign of the enthalpy change should be?

You have to think about whether or not energy is being released by the system into the surroundings (exothermic), or whether the system is taking energy up from the surroundings (endothermic).You have to put energy into the system to break bonds and energy is released when new bonds form. So, if the amount of energy released when the new bonds form is less than what was required to break the bonds (endothermic), a net amount of energy had to be taken in from the surroundings in order to achieve this. Hence the amount of energy in the system has increased. In this case the enthalpy change of the system is positive, so you use a plus sign. When the opposite is true, when more energy is released when the new bonds were formed than was required to break the old bonds (exothermic), the amount of energy contained within the system decreases, as the excess energy is released to the surroundings. In this case the enthalpy change of the system is negative and so a minus sign is used.