I see a lot of advice on this subject:

Break up the mortgage payment into two, then pay every 4 weeks, rather than once per month.

It's a little trick to keep you strict on your budget, and ends up putting more toward your mortgage than paying once per month.

It helps you pay one extra payment per year.

12 payments vs 26 bi-weekly (half) payments (52 weeks in a year, divided by 2)

26 half payments = 13 full payments

One extra payment per year.

How much does it actually help?

In this example we're going to use a $400,000 loan amount, the last average rate I saw on mortgage news daily's website (6.5%) and a 30 year term.

Here's a regular schedule if you were to pay once a month

Part 1: Regular 30 year schedule

This shows what the balance will be at the end of each year:

Year	Balance
1	$395,529
2	$390,758
3	$385,668
4	$380,238
5	$374,443
6	$368,261
7	$361,664
8	$354,626
9	$347,117
10	$339,104
11	$330,555
12	$321,433
13	$311,700
14	$301,316
15	$290,236
16	$278,414
17	$265,800
18	$252,342
19	$237,982
20	$222,661
21	$206,313
22	$188,871
23	$170,260
24	$150,403
25	$129,216
26	$106,610
27	$82,491
28	$56,756
29	$29,297
30	$0.00

By the end of 30 years you'll have paid $510,177.95 in interest

Part 2: bi-weekly schedule

Year	Balance
1	$392,913
2	$385,351
3	$377,282
4	$368,672
5	$359,484
6	$349,680
7	$339,219
8	$328,056
9	$316,144
10	$303,433
11	$289,869
12	$275,396
13	$259,952
14	$243,472
15	$225,887
16	$207,122
17	$187,099
18	$165,733
19	$142,933
20	$118,605
21	$92,645
22	$64,943
23	$35,383
24	$3,841
25
26
27
28
29
30

By the end of this one, you'll pay about $392,682 total interest

Part 3 side by side to see the advantage

Year	Monthly	bi-weekly
1	$395,529	$392,913
2	$390,758	$385,351
3	$385,668	$377,282
4	$380,238	$368,672
5	$374,443	$359,484
6	$368,261	$349,680
7	$361,664	$339,219
8	$354,626	$328,056
9	$347,117	$316,144
10	$339,104	$303,433
11	$330,555	$289,869
12	$321,433	$275,396
13	$311,700	$259,952
14	$301,316	$243,472
15	$290,236	$225,887
16	$278,414	$207,122
17	$265,800	$187,099
18	$252,342	$165,733
19	$237,982	$142,933
20	$222,661	$118,605
21	$206,313	$92,645
22	$188,871	$64,943
23	$170,260	$35,383
24	$150,403	$3,841
25	$129,216
26	$106,610
27	$82,491
28	$56,756
29	$29,297
30	$0.00

So here's the gist of it

You'll pay it off almost 6 years faster

You'll save about $117,000 total interest

After 5 years you'll have a ~$15,000 balance advantage.

Keep in mind that some mortgage servicers will not allow you to pay it in halves. If that's the case, set the first half aside, on the bi weekly schedule, and pay the lender on the second half.