I just don't know how to defend my other bases. I'm a terran player and unless it's an easy to get to island expansions which can only be reached by air, I seem to have a really hard time defending anything but my main base (with siege tanks and marauders usually). Is it just a case of needing to do more scouting so you know where the enemies going to attack and thus avoid splitting your forces? Have no clue what else I could do other than that (scouting is an area I generally need to improve on). 
 
Currently playing the AI on Insane - Was having a cake walk once I'd worked out how to harass properly with a load of reapers on the lower difficulties but now the early resource game seems to always be going the computer's way and my defences are in a bad shape.

If its like most maps where your natural expansion is right next to the ramp leading up to your main, post seige tanks along the ridge of main base plateau to take advantage of the height and blast any ground forces marching towards your expansion.  But as Fragstoff said, you'll want to keep the bulk of your defense mobile.

Largely, this is just the way Terran is. Terran is usually less mobile than the other races, and will have a harder time expanding around the map. On the other hand, they're excellent for fortifying and protecting small areas.

Terran don't need to expand quite as much as the other races, due to being able to MULE and their units being so powerful in a defensive position. 
 
This isn't to say you can afford to not expand.  Just don't worry as much about doing it quicker then a Toss or Zerg player.  When you try to play their game, you will lose.  Force them to play your game.  Make your base impenetrable, then roll over them with the correct selection of units.

You're Terran. The most turtling race in the game. Try Planetary Fortresses, bunkers, turrets, and siege tanks.

" Terran don't need to expand quite as much as the other races, due to being able to MULE and their units being so powerful in a defensive position.  This isn't to say you can afford to not expand.  Just don't worry as much about doing it quicker then a Toss or Zerg player.  When you try to play their game, you will lose.  Force them to play your game.  Make your base impenetrable, then roll over them with the correct selection of units. "

" @SamDrugbringer said:

" Terran don't need to expand quite as much as the other races, due to being able to MULE and their units being so powerful in a defensive position.  This isn't to say you can afford to not expand.  Just don't worry as much about doing it quicker then a Toss or Zerg player.  When you try to play their game, you will lose.  Force them to play your game.  Make your base impenetrable, then roll over them with the correct selection of units. "

I always try to expand just after I've pushed back a major assault on my main - that way I know they likely don't have too many units with which they could send my way again.  Also, would a sensor tower's detection radius help increase the range on my siege tanks? They can fire past their initial line of sight right? Or would I need other units with a view of the area to take advantage of this?  Thanks for all the help so far - never played starcraft  before and I could barely beat the AI on normal at the start of last week - massively improving since then (mainly with regard to multitasking and keeping my resource count below 200 at all times unless I'm going for the higher tiered units). Never been any good at RTS games - also always found them slow and unexciting, filled with too much waiting for units/minerals but now I realise that was just down to the style of play I had. It's so hard to keep up with everything here... "

Planetary Fortress, a couple of siege tanks and a few AA turrets should keep them safe, or at least make the losses to great for people to not bother attacking them.

I have to protest, you should try to avoid building p lanetary fortresses unless you absouletely have to. 
 
The extra mules are ALWAYS worth it.  ALWAYS.

That depends. Usually your first expansion should be an Orbital Command, but since Terran is so immobile when they go mech (and especially against Zerg, bio or mech), it would be advisable to turn the third Command Center into a Planetary Fortress so it can hold on for longer while you try to move your forces to it. The third base should, in most cases, force you to be spread thinner than you would like, so any extra time you can buy to keep it alive while you rush your troops there is great. There are possible exceptions when you have a number of bases really close to your main, such as Delta Quadrant. Most others though, I would go PF myself, but to each their own I suppose.

I just seem to have a hard time building up enough defences to push back any attacks whilst also creating enough reapers/marauders to properly harass the enemy and stop them claiming all the expansions. I feel like I have my early game down pretty well but my late game economy just isn't producing enough resources for me to stay in the game. Always find myself on the back foot.  
 
How many units would make for a decent strike force to disrupt the enemy's economy? I keep trying to build around 10 reapers to take out bases quickly but that takes far, far too long.  That tactic's probably a stay over from my AoE days when I liked to turtle up, tech everything and then build a massive force to steam roll my opponents. Not used to micro-ing at all so have no idea how many units I need for each situation.

" @Vitor:  Its generally agreed upon that aside from very early game harassment, reapers just aren't worth it due to how fragile they are.  A group of banshees would fair better in mid game when you want to abuse workers at an unprotected base. "

" @Vitor:  You don't really need to create 3 distinct armies for attacking, defending and harassment. For example, an intantry ball backed by medivacs and seige tanks is generally quite good for attacking. Repositioned, it's also quite good for defending. Or, load some of those infantry into one of those medivacs, and you've got your harassment force.  Now for late game harassing, it really depends what you want to do. If you want to take out entire bases quickly, you're probably best just running your infantry ball over there, killing it, and running back before your opponent responds. For scouting the map and killing workers at an undefended expansion, a group of 4 hellions is all you really need. "