It is important that your pup gets used to being groomed from the outset - struggling with a 150 pound fully-grown Saint who doesn't want his tummy groomed is no laughing matter!
With young Saints, a soft brush and a comb are sufficient. Go over all of the dog with the soft brush making sure to brush in the direction that the fur lies, and finish off any stubborn tangles with the comb. Do the ears, tail, neck and the back of the legs thoroughly - these are the bits that will get tangles as your dog grows.
For fully grown Saints, I have found the best grooming products are the slicker brush and the rake type brushes.
You can either let the dog lay down for the whole process and roll him over to do the different sides and underneath, or you can teach him to stand still. I prefer mine to lie down as grooming a fully grown Saint Bernard can take some time and it's a bit unfair to expect them to stand in the same position for too long.
To give you some idea, this is how I go about grooming one of mine :
I lie the dog down on his side and with the slicker brush, go down the whole side of the dog in steady and smooth brush strokes. You will need to disentangle the pins of the slicker brush several times and I have found that the easiest way to do this is to put a comb through it. Thoroughly brush the one side of the dog (use the rake if your dog is moulting, it's an excellent tool for getting out that fine undercoat) taking more time on the long hair at the back of the legs (front and rear). Now, roll the dog over onto his back and do the same with his tummy, don't be too harsh here as the hair around the groin is quite thin and a slicker brush used too roughly will scrape the skin. Brush thoroughly around the underneath of the neck and use the comb to get out any tangles around this area. Roll your dog over onto his other side and brush as before. The tail can be done now if you wish - go through it with the slicker or rake first and then with the comb.
Now, sit the dog up and do his head, make sure you do behind the ears as the fine, long hair here can become easily tangled, do all over his head (take care around his eyes) and thoroughly brush his bib giving it a final going over with the comb. Go down the centre of the back with the rake or the slicker brush.
You should now have a fairly respectable looking dog - to finish the job off completely use dog ear wipes to clean out any old wax in the ears (don't dig too deep with these or you will hurt him) and dog eye wipes to clean off any eye discharge. You can also trim the claws if you wish but only if you know what you are doing - the claw needs to be cut about 1/4 of an inch below the "quick" - the fine blood vessel that runs down each claw and can be seen as a darker, red line inside the claw itself - if you cut too far back you will be amazed at how much it bleeds and your Saint Bernard may not be so keen to give you his paw the next time they need a trim!
I have also found that the fur on the pad of the paw needs regular trimming as it can become matted and tangled, especially if your dog is walked through muddy fields and forests! Use a good pair of scissors and just trim back the fur to the bottom of the pad, the fur in between the toes may require similar attention.
You can now bath your dog if you so wish, it is always best to groom before a bath; water in the coat will only make worse any already present tangles and matts. Bath time should be fun for your dog - mine love to jump in the bath, even when it's not bath time! - use the shower attachment, thoroughly wet your dog and use a good dog shampoo. A word of wisdom here - if you leave the head til last, your dog is less likely to shake himself as you are washing him! When washing the head be careful not to get shampoo in his eyes or down his ears. Always rinse the dog thoroughly afterwards and give him an attempt at being dried with a towel - this is usually a waste of time, even large bath towels become soaked within seconds of going near an even slightly damp Saint Bernard - and then either leave him to dry naturally or give him a blow dry with the hairdryer on a low setting. You will probably find that your dogs hair sticks up for fine weather for a few days after a bath, don't worry, it will soon settle and lie flat again.
Grooming long-haired Saint Bernard's is a regular chore and should be done at least once a week if not more. You will find that the more regularly you do the grooming, the less time it takes you when you want to do a full pampering session.