Fours, fives, and sixes are the worst offs because they are the suits with the ten counters.
Loosing a single ten count does not set a bid of 31.
To set a bid of 36 or less the opposing team needs at least a ten count or a five count and two tricks. At 37 you cannot lose any count dominoes. If you have a reasonably good hand, bid 36 to make it hard for the opposing team to raise you.
If you have lots of count you can bid small (e.g., 30) to indicate to your partner that you have a helping hand. Conversely, if your partner bids small (30 or 31) then they would probably appreciate you raising them if you can.
When deciding on your bid you can rely on your partners hand to help you make your bid.
Having your double in front of your offs can help a lot (esp when playing 84).
You generally want to pull the trumps out of your opponents hands as quickly as possible.
If you have lots of trumps and doubles but a bad off you can sometimes minimize your loses by opening with it. This can help prevent your opponents from stacking count on it that isn't in the same suit.
If you are playing with the Small End rules you can sometimes get rid of a bad four, five, or six off by calling the other end on the opening play.
If you don't have the double of your trumps or one of your trumps is in jeopardy you want to try to get your partner in the lead. Open with a small off.
You can sometimes trick your opponents into giving you count by opening with a good, but non-double trump. If your opponents have the bid and open with a non-double don't give them more than a five count unless they open with a small trump (if they can walk a small trump on the opening play then you probably aren't going to set them so playing your ten count isn't going to hurt you).
Get rid of offs as soon as possible to avoid being stuck with them towards the end when all the count is being played.
Try to keep a trump for the last trick on bids of 30-35 to make sure you can catch any count your opponents might be holding.
When playing an 84 bid you want to try to walk your offs whenever possible. Having a double in front of your offs is almost a must.
Hit them hard. If you have all the count then play it and hope you either hit their off or your partner can catch it.
If the trick can be caught by your team then play count on it unless you are sure that the other team would trump it. You usually only have only a one or two trick window in which to set your opponents so you have to take the chance when you get it.
Get rid of any domino suits you can early in the hand so you can play count dominoes for your partner to catch or trumps to catch count, should that suit happen to be played.
You can sometimes trick your opponents into believing they have all the trumps by giving away a small count trump domino. For example, if fives are trumps and you have two, and some other five, and on your play your partner did not have any trumps then you can sometimes play your to trick the other team into thinking you don't have another trump. This can work for other 5 count dominoes as well.
Track only the dominoes you can hold for to help you decide which dominoes to hold on to. This is most important when playing 84, etc since the older tricks cannot be seen.
Setting Nel-O is usually pretty easy unless you or your partner have lots of big dominoes. Playing the will almost always set someone that isn't experienced at Nel-O.