General Motors Corp. sales slid 20.3% in August, but the auto maker draws cold comfort from its highest monthly sales total and best market-share performance so far this year.

GM dealers delivered 307,379 vehicles on a daily basis, compared with 385,529 in like-2007 (with 27 selling days in both years), according to Ward’s data. Industry-wide, the seasonally adjusted annual rate stood at about 14 million units in August, which marked the first increase since the monthly metric hit 15.8 million in January.

“We’re pretty pleased and encouraged by our August sales results,” says Mark LaNeve, vice president-sales, service and marketing. “It was our best month of 2008 in total sales, retail sales, fleet sales and in market share. It was far and away our best month of the year.”

GM estimates it finished August with 24.5% of the U.S. market, boosted by an employee-pricing incentive program the auto maker intends to continue through September with the addition of some previously excluded ’09 models.

While the incentive scheme is one that dealers typically dislike because it cuts into their margins and tends to pull future buyers into showrooms early, LaNeve characterizes employee-pricing as “a strong, efficient play for us.”

The program already has pushed GM ahead by 20,000-25,000 units over its trailing 90-day sales pace, and the sales executive expects much less of a payback in the coming months than during the auto maker’s last employee-pricing event in 2005.

“The market has been operating at such a depressed level, I think with any improvement in the market we won’t have any payback at all,” LaNeve tells journalists and Wall Street analysts during a monthly conference call today.

He also maintains dealers welcomed the program. “I think they were so desperate to do some business they were delighted they saw a lot of customers and wrote some deals. They appreciated us being aggressive in the market.”

LaNeve downplays the expense of employee-pricing for all, calling GM’s incentive spend in the month “stone-cold flat with July” and up $700 compared with year-ago – an increase on par with principle competitor Toyota Motor Corp.

“It was effective the first time we ran it in ’05, and it has proven to be effective again,” he says. “I’m not sure it’s something you want to go to the well with month-in, month-out, but…we think it was a good play, well executed.”

GM keeps a pledge to step back from leasing with residual values still frightfully low on pickups and SUVs, a move that also helped keep its incentive spend relatively low.

“We did almost no leasing support in the month of August,” LaNeve adds. “Not that we don’t like leasing as a mechanism to put consumers into cars. It just became cost-prohibitive due to a number of dynamics we hope are not permanent.”

Key performers for the auto maker included the Cadillac CTS, which jumped 51%, and the Chevrolet Impala, which saw a 21.4% sales increase, according to Ward’s data. GMC Acadia deliveries climbed 25%, and Chevy Avalanche sales rose 9.4%, which represents the largest spike for a fullsize truck this year.

GM says it sold 80,000 fullsize trucks in August, for its best result in the segment in 12 months, and 22,000 fullsize SUVs, for the best month for that segment this year. The additional pickup and SUV sales resulted from a combination of ’08-model clear-outs and lower prices at the pump.

Mike DiGiovanni, GM’s chief sales analyst, says the additional truck sales, which had tanked since the spike in gasoline prices earlier this summer, helped push the auto maker’s inventory down to 736,000 units from 747,000 in July.

That’s a 202,000-unit drop from year-ago, marking GM’s second-leanest month-ending inventory number on record.

DiGiovanni says the inventory number, combined with lower oil prices and other positive economic indicators, hint the industry may have hit bottom.

“There are still some serious issues in the credit markets, but we do see some things to reinforce that the economy may be starting to gain some traction,” he says, while also citing a firmer dollar; a housing market that appears near the bottom of its trough; stabilizing consumer sentiment and manufacturing rates; and continued strength in exports.

Says DiGiovanni: “We feel we may be seeing the likelihood of further economic deterioration mitigating, and perhaps we’re starting to turn the corner.”