A very important point to remember at all times is that you need to have a more aggressive Internal SLA vs. the one that you are offering to your customers.
I know it sounds self evident doesn't it, but there are no end of organizations that I've dealt with where customers are offered a 4hr SLA on a 24/7 basis and the engineers that can actually fix the problem are either unavailable till the next business day or NOT even on call!!!
Let me state this once again and very clearly so that there is NO CONFUSION ... If you are offering your customers an SLA of 'X Hours' and your Engineering (or Development or Project Management or ... etc...) team is only offering you an SLA of 'X + Y Hours' ... YOU WILL LOSE MONEY and YOU WILL LOSE CUSTOMERS!!!
It is imperative that your internal SLA be better than the one you are offering to your customers and you need to ensure that your Sales team and Senior Management are both on board with this.
Remember, also, that this must go all the way up the chain ... your Engineering team has agreed to an internal SLA of 'X – Y Hours' (woohoo!! That will solve 80% of your problems) but the Development team is only offering them an SLA of 'Z' (assume 'Z' is a multiple of 'X + Y') ... for those 20% of customers and problems that cannot be solved by your Tier 2 (Engineering team in this example) group ... you are still going to be in trouble.
The question, now becomes how much are you and your company willing to invest in protecting yourself from that 20%? Just like everything else there are things you can and cannot do, and you need to decide what your investment will be to give you the best "bang for your buck".