Since many devices, especially the most useful ones, have a target number of 5 most of these devices are only going to have a reliability of 1. The only way to increase this is with bonuses from studying advanced devices or repeat attempts. The bonuses for studying advanced devices varies and will be detailed in that scenario's description. Most are +1, but can be as high as +3.
Repeat attempts to improve a device's reliability give a +1 per each previous success.
Sacrificing extra invention dice.
If neither of the above are availabile, the PC can sacrifice extra dice to boost his chances of success. For each extra die sacrificed in this way, the PC can add +1 to his roll.
Example: A PC has studied an ancient Martian text on the properties of Liftwood and the detailed text gives him a +1. Later after amassing the required level of 42 in Biochemistry, he looks at the intimidating target number of 5 to successfully create Artificial Liftwood. At this point he still has one die of Chemistry and two Intellect dice. If he decides to make maximum effort, he can roll 1D6+3 to invent the device.
The one Chemistry invention die, +2 from the 2 Intellect dice, and +1 from the Martian text book. Rolling a 4+3=7 he succeeds and has a reliability of 2.