Merging from a freeway entrance

Use the slip road (on-ramp) to get up to the same speed as traffic on the freeway or motorway as quickly as possible as this gives you more options. When you are travelling at the same speed your entrance will be less disruptive to traffic flow. Time your entrance so that you emerge from the on-ramp into a sufficiently large gap. You should be signalling your intention to merge and drivers will usually oblige.

Don't, whatever you do, stop at the end of the on-ramp. This is extremely dangerous as you then stop everyone else behind you and now every driver has to try to get on the motorway at a speed much less than the flow of traffic. This situation is likely to cause traffic congestion as everyone in the left lane slows to a crawl to accommodate your poor merging effort, forming a jamiton or phantom traffic jam further back on the motorway. 

Merging like a zip (i.e. take turns), where vehicles from the on-ramp slot in between gaps in the traffic flow, is the best solution to keep traffic moving. 

Just before you move over to your right to enter the motorway, check in your blind spot. Be careful of traffic that might brake ahead of you - you really need to be able to look ahead, behind and to your right at the same time, which is impossible, and that's why it's important you have judged your speed and picked a slot to merge into, and that you are following a safe distance behind the vehicle in front.

If your car has blind spot monitoring, this will help you somewhat, but it depends on the angle of the slip road entrance and how sensitive the sensors are on the car. These detectors tend to work when two lanes are running parallel with one another rather than converging at an angle, so don't assume that if you don't have the blind spot monitor warning light or audible warning that it's safe to change lanes; always check over your shoulder.

One type of vehicle to be careful of is long vehicles towing a trailer on a drawbar. The trailer could be in your blind spot and you might see (out of the corner of your eye) what you think is the back of the truck and start moving across. 

Motorcyclists should be aware of being in the wind shadow of a large vehicle as they emerge onto the motorway.

Here's a video from WA about general merging in traffic:

Merging from the hard shoulder

If you have to stop on the hard shoulder to change a tyre or check a load then you will need to get back up-to-speed before merging on the motorway. DON'T pull onto the motorway and then try to get up to speed. Use the hard shoulder to build your speed up so that it matches that of the vehicles already on the motorway, then choose an appropriate gap, indicate for three seconds and move into it if it's safe to do so.

Be careful on the hard shoulder. Be aware that other vehicles might be parked on it, there could be loose gravel and there might be a ridge of tarmac between the shoulder and the lane you're moving into which will make it more difficult to steer onto it.