|First, a little background:
I use a cardboard table protector with a grid marked off in inches on my table; mine also has true bias marked on it. I can't imagine sewing without it anymore! If you don't have one, you can still use this tip but you'll have to slide the fabric rather than pivoting it to align the patten's grainline with the fabric grain.
This having been said, here's my tip:
When I use a pattern, I extend the grainline marking (using a yardstick and a pencil) all the way to the edges of the pattern pieces. I pick two spots along the pattern's grainline near the edges of the pattern piece, usually a few inches from the edge, and put a one-inch piece of tape on each spot. I also put a smaller piece of tape at the halfway point of the grainline.
The tape I use is removable Scotch tape but you can use masking tape or any type you like; it's just there to reinforce the pattern so it doesn't tear.
I lay out the fabric after having made sure the grain is straight. There is a great link in the Knowlegebase section of this site on preparing woven fabric for use and a great tip from Mary T in the Tips section about finding and marking the grain of knits, if you'd like to find out how to straighten the grain. To find either of these sets of instructions, search each section using the word "grain".
I align the fabric to the markings on my cardboard liner and pin the fabric to the cardboard along the selvedges and the cut edge. If you don't have a pinnable liner, you can lay out the fabric and use heavy weights or removable tape on the edges to keep it from sliding.
I don't let the fabric hang off the edge of the table as it makes it hard to keep the fabric true. I usually make a neat little pile with the extra fabric at the edge of the table where it would have been hanging off.
I lay out the pattern pieces and roughly true them to the grain.
To begin securing each pattern piece, I pin through the center piece of tape, the pattern and the fabric into the cardboard underneath. I put this first pin in perpendicularly to the table (that is, straight down). I use this pin as a pivot to true the pattern's grainline to the fabric's grain. If you don't have a pinnable liner, slide the pattern gently across the fabric rather than pivoting.
I align the pattern's grainline to the fabric grain by measuring from one end of the pattern grainline to the edge of the fabric where it has been secured to the cardboard underlay, then I do the same with the other end of the pattern grainline. I compare the measurements. If they are different, I calculate the difference. (I use the metric/millimeter side of a yardstick as it's easier for me to do the math and, since the unit of measurement is smaller, it's easier to get the grainline exactly placed.) I then halve the difference and pivot the pattern up or down that amount. I do another quick set of measurements to make sure that the pattern's grainline is indeed aligned and then I go on to securing the pattern. If I think I may make that pattern again, I lightly record the final measurement in pencil along the grainline; that way, I don't have to go through the whole process again.
Once the pattern has been trued to the grain, I pin one end of the pattern down through one of the larger piece of tape. I pin through the tape, pattern, and fabric into the cardboard, pinning at an angle through the tape. Then I pin the other end the same way.
I check that the grain is still true and then put in another pin at each end of the pattern, crossing the pins for each piece of tape. That is, one pin goes in at a perhaps a 35 degree angle and the other goes in at a 145 degree angle; they are about half or three quarters of an inch apart and are both pinned in through the grainline. Having the pins cross keeps the pattern from shifting on the fabric and the fabric from shifting on the underlay.
I use pattern weights (cat food cans and washers) to secure the rest of the pattern piece.
To do this whole process (extending the grainline, taping the pattern, and pinning through the tape) takes a few minutes but it saves me so much time because I don't have to worry whether my pattern has shifted off grain!